Read a mailbox read-only, in a client small enough to audit (#246)

internal/email is the reading half of the email reader: a ~200-line IMAP
client (LOGIN, EXAMINE, UID SEARCH SINCE, UID FETCH BODY.PEEK, LOGOUT), a
MIME-to-plaintext converter, and a header-only junk filter.

Two protocol choices are the design, not shortcuts. EXAMINE instead of
SELECT means the session is read-only at the protocol level, so no command
in it can flip a flag or expunge anything by mistake. BODY.PEEK instead of
BODY means reading a message does not mark it \Seen — Maven reads his mail
and leaves no trace of having done so, and the unread state in his own
client stays his.

Hand-rolled rather than go-imap because this is the one path that holds his
mailbox credential and reads his private mail: five commands with no
dependencies is auditable in a sitting. No IDLE and no cleartext/STARTTLS
either — an option to send his password over a plain socket is an option to
get it wrong once.

Junk is decided by headers alone, before any model is involved:
List-Unsubscribe/List-Id, Precedence: bulk, Auto-Submitted, the spam
headers, and Gmail's own category labels. Sender lists and subject keywords
are deliberately absent — they age badly and they would put his contacts in
a config file. A junk verdict only means "do not spend the model on this";
nothing is deleted and no server flag is touched.

Nothing here logs a body, a subject or an address, the junk reason names a
header rather than content, and an undecodable charset degrades to
headers-only instead of feeding the model mojibake. Verified against
recorded .eml fixtures and an in-process fake IMAP server.

internal/email/fetch.go

@@ -0,0 +1,96 @@
+package email
+
+import (
+	"fmt"
+	"time"
+)
+
+// FetchSince is the whole read path in one call: connect, log in, examine the
+// mailbox read-only, list what arrived since a date, fetch and parse the ones
+// the caller has not seen, log out.
+//
+// It is a function rather than a long-lived object because a mail poller should
+// not hold an authenticated session (and therefore his credential in a live TLS
+// state) between polls. Connect, read, drop.
+//
+// skip decides which UIDs are already known — the poller's seen-set. max bounds
+// one poll: a mailbox that received 400 messages overnight must not turn into
+// 400 LLM calls, and the newest max are the ones a task could still be hiding
+// in. Junk messages are returned too, flagged, so the caller can mark them seen
+// without a second protocol round.
+type FetchSince struct {
+	Addr    string // host or host:993
+	User    string
+	Mailbox string // e.g. "INBOX"
+	Timeout time.Duration
+	Since   time.Time
+	Max     int
+	Skip    func(uid uint32) bool
+
+	// dial is the connection seam. nil means Dial (implicit TLS); the tests set
+	// it to an in-process fake. Unexported so no configuration path can point
+	// the reader at a non-TLS transport.
+	dial func(addr string, timeout time.Duration) (*Conn, error)
+}
+
+// Run performs one read. password is passed here, not stored in the struct, so
+// the configuration of a mailbox and the secret for it are never the same value
+// sitting in the same place.
+func (f FetchSince) Run(password string) ([]Message, error) {
+	if f.Addr == "" || f.User == "" || f.Mailbox == "" {
+		return nil, fmt.Errorf("email: mailbox not configured (addr/user/mailbox)")
+	}
+	dial := f.dial
+	if dial == nil {
+		dial = Dial
+	}
+	c, err := dial(f.Addr, f.Timeout)
+	if err != nil {
+		return nil, err
+	}
+	defer c.Close()
+	if err := c.Login(f.User, password); err != nil {
+		return nil, err
+	}
+	defer c.Logout()
+	if err := c.Select(f.Mailbox); err != nil {
+		return nil, err
+	}
+	uids, err := c.SearchSince(f.Since)
+	if err != nil {
+		return nil, err
+	}
+
+	// Newest UIDs first — IMAP hands them back ascending, and when Max clips the
+	// list the recent mail is what matters.
+	wanted := make([]uint32, 0, len(uids))
+	for i := len(uids) - 1; i >= 0; i-- {
+		if f.Skip != nil && f.Skip(uids[i]) {
+			continue
+		}
+		wanted = append(wanted, uids[i])
+		if f.Max > 0 && len(wanted) >= f.Max {
+			break
+		}
+	}
+
+	out := make([]Message, 0, len(wanted))
+	for _, uid := range wanted {
+		raw, err := c.Fetch(uid)
+		if err != nil {
+			// One unreadable message does not abandon the poll; the rest of the
+			// mailbox is still worth reading. The error names the UID, not the
+			// message.
+			return out, fmt.Errorf("email: fetch uid %d: %w", uid, err)
+		}
+		if len(raw) == 0 {
+			continue // vanished between SEARCH and FETCH
+		}
+		msg, err := ParseMessage(uid, raw)
+		if err != nil {
+			continue // unparsable headers — nothing to review, skip silently
+		}
+		out = append(out, msg)
+	}
+	return out, nil
+}

internal/email/fetch_test.go

@@ -0,0 +1,50 @@
+package email
+
+import (
+	"net"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestFetchSinceRun(t *testing.T) {
+	mk := func(subject string) string {
+		return "Subject: " + subject + "\r\nContent-Type: text/plain; charset=utf-8\r\n\r\nbody\r\n"
+	}
+	f := &fakeIMAP{
+		uids: []uint32{1, 2, 3},
+		msgs: map[uint32]string{1: mk("one"), 2: mk("two"), 3: mk("three")},
+	}
+	fs := FetchSince{
+		Addr: "mail.example:993", User: "kami", Mailbox: "INBOX",
+		Timeout: 5 * time.Second,
+		Since:   time.Date(2026, 7, 30, 0, 0, 0, 0, time.UTC),
+		Max:     2,
+		Skip:    func(uid uint32) bool { return uid == 3 },
+		dial: func(addr string, timeout time.Duration) (*Conn, error) {
+			cli, srv := net.Pipe()
+			go f.serve(t, srv)
+			return NewConn(cli, timeout)
+		},
+	}
+	msgs, err := fs.Run("secret")
+	if err != nil {
+		t.Fatalf("run: %v", err)
+	}
+	// Newest first, the already-seen UID skipped, Max respected.
+	if len(msgs) != 2 {
+		t.Fatalf("got %d messages, want 2: %+v", len(msgs), msgs)
+	}
+	if msgs[0].Subject != "two" || msgs[1].Subject != "one" {
+		t.Errorf("subjects = %q,%q, want two,one (newest first)", msgs[0].Subject, msgs[1].Subject)
+	}
+	if strings.Contains(strings.Join(f.cmds, " "), "UID FETCH 3") {
+		t.Error("a skipped UID must not be fetched again")
+	}
+}
+
+func TestFetchSinceRequiresConfig(t *testing.T) {
+	if _, err := (FetchSince{}).Run("secret"); err == nil {
+		t.Fatal("an unconfigured mailbox must not be read")
+	}
+}

internal/email/imap.go

@@ -0,0 +1,280 @@
+package email
+
+import (
+	"bufio"
+	"crypto/tls"
+	"fmt"
+	"io"
+	"net"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// A minimal IMAP4rev1 client — LOGIN, SELECT, UID SEARCH, UID FETCH with
+// BODY.PEEK, LOGOUT, and nothing else.
+//
+// Why hand-rolled instead of go-imap: the whole surface Maven needs is five
+// commands, and this is the one code path that holds his mailbox credential and
+// reads his private mail. A ~200-line client with no dependencies is auditable
+// in one sitting; a general-purpose IMAP library is a much larger amount of
+// code doing much more than we asked, in the most sensitive place in the tree.
+// If IDLE, CONDSTORE or server-side threading ever become worth having, that
+// trade should be re-made deliberately.
+//
+// BODY.PEEK[] rather than BODY[] is load-bearing: Maven reads his mail and must
+// leave no trace of having done so. Reading a message here does not mark it
+// \Seen, so the unread state in his own mail client stays his.
+
+// DefaultIMAPPort — implicit-TLS IMAP. There is no cleartext and no STARTTLS
+// path in this client: an option to send his password over a plain socket is an
+// option to get it wrong once.
+const DefaultIMAPPort = "993"
+
+// Conn — one authenticated IMAP connection. Not safe for concurrent use; the
+// poller drives one connection at a time.
+type Conn struct {
+	rwc     io.ReadWriteCloser
+	r       *bufio.Reader
+	tag     int
+	timeout time.Duration
+}
+
+// Dial opens an implicit-TLS connection and reads the server greeting.
+func Dial(addr string, timeout time.Duration) (*Conn, error) {
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		host, addr = addr, net.JoinHostPort(addr, DefaultIMAPPort)
+	}
+	d := &net.Dialer{Timeout: timeout}
+	// ServerName is set from the host we asked for: certificate verification is
+	// the only thing standing between his password and a MITM on the way out.
+	c, err := tls.DialWithDialer(d, "tcp", addr, &tls.Config{ServerName: host, MinVersion: tls.VersionTLS12})
+	if err != nil {
+		return nil, fmt.Errorf("email: dial %s: %w", addr, err)
+	}
+	return NewConn(c, timeout)
+}
+
+// NewConn wraps an already-open stream (the tests speak IMAP over a pipe) and
+// consumes the greeting.
+func NewConn(rwc io.ReadWriteCloser, timeout time.Duration) (*Conn, error) {
+	c := &Conn{rwc: rwc, r: bufio.NewReaderSize(rwc, 64<<10), timeout: timeout}
+	line, err := c.readLine()
+	if err != nil {
+		return nil, fmt.Errorf("email: greeting: %w", err)
+	}
+	if !strings.HasPrefix(line, "* OK") && !strings.HasPrefix(line, "* PREAUTH") {
+		c.rwc.Close()
+		return nil, fmt.Errorf("email: server refused connection: %s", line)
+	}
+	return c, nil
+}
+
+func (c *Conn) Close() error { return c.rwc.Close() }
+
+// Login authenticates with LOGIN. The password is passed as an argument and
+// never stored on the Conn: nothing in this package keeps a credential alive
+// past the command that uses it, so no struct dump or panic trace can carry it.
+func (c *Conn) Login(user, pass string) error {
+	// The command line itself is never logged (see exec) — a LOGIN line IS the
+	// credential.
+	if _, err := c.exec(fmt.Sprintf("LOGIN %s %s", quote(user), quote(pass))); err != nil {
+		return fmt.Errorf("email: login: %w", err)
+	}
+	return nil
+}
+
+// Select opens a mailbox read-only. EXAMINE, not SELECT: read-only at the
+// protocol level means no command in this session can change a flag, expunge a
+// message, or move anything, even by mistake.
+func (c *Conn) Select(mailbox string) error {
+	if _, err := c.exec(fmt.Sprintf("EXAMINE %s", quote(mailbox))); err != nil {
+		return fmt.Errorf("email: examine %s: %w", mailbox, err)
+	}
+	return nil
+}
+
+// SearchSince returns the UIDs of messages received on or after since. An
+// unlimited search is not offered: the first poll against a years-old mailbox
+// would otherwise fetch everything and hand a decade of mail to the model.
+//
+// The IMAP SINCE key has date granularity (and compares the server's internal
+// date), so the result can include messages slightly older than since. The
+// caller dedupes by UID anyway, so a wider window costs one extra fetch.
+func (c *Conn) SearchSince(since time.Time) ([]uint32, error) {
+	cmd := fmt.Sprintf("UID SEARCH SINCE %s", since.Format("2-Jan-2006"))
+	lines, err := c.exec(cmd)
+	if err != nil {
+		return nil, fmt.Errorf("email: search: %w", err)
+	}
+	var uids []uint32
+	for _, l := range lines {
+		rest, ok := untagged(l, "SEARCH")
+		if !ok {
+			continue
+		}
+		for _, f := range strings.Fields(rest) {
+			n, err := strconv.ParseUint(f, 10, 32)
+			if err == nil {
+				uids = append(uids, uint32(n))
+			}
+		}
+	}
+	return uids, nil
+}
+
+var literalSize = regexp.MustCompile(`\{(\d+)\}$`)
+
+// Fetch returns the raw RFC 5322 bytes of one message, by UID.
+//
+// Returns (nil, nil) when the UID no longer exists — a message he deleted
+// between SEARCH and FETCH is normal, not an error.
+func (c *Conn) Fetch(uid uint32) ([]byte, error) {
+	tag := c.nextTag()
+	if err := c.send(fmt.Sprintf("%s UID FETCH %d (BODY.PEEK[])", tag, uid)); err != nil {
+		return nil, err
+	}
+	var raw []byte
+	for {
+		line, err := c.readLine()
+		if err != nil {
+			return nil, fmt.Errorf("email: fetch %d: %w", uid, err)
+		}
+		if done, err := c.tagged(tag, line); done {
+			if err != nil {
+				return nil, fmt.Errorf("email: fetch %d: %w", uid, err)
+			}
+			return raw, nil
+		}
+		m := literalSize.FindStringSubmatch(strings.TrimSpace(line))
+		if m == nil {
+			continue
+		}
+		n, err := strconv.Atoi(m[1])
+		if err != nil {
+			continue
+		}
+		buf := make([]byte, n)
+		if _, err := io.ReadFull(c.r, buf); err != nil {
+			return nil, fmt.Errorf("email: fetch %d: literal: %w", uid, err)
+		}
+		if raw == nil {
+			raw = buf
+		}
+	}
+}
+
+// Logout ends the session politely. A failure is not worth reporting — the
+// connection is being closed either way.
+func (c *Conn) Logout() {
+	_, _ = c.exec("LOGOUT")
+}
+
+// ---- protocol plumbing -----------------------------------------------------
+
+func (c *Conn) nextTag() string {
+	c.tag++
+	return fmt.Sprintf("a%03d", c.tag)
+}
+
+// exec sends one command and returns the untagged response lines.
+//
+// Neither the command nor the response is ever logged here. LOGIN goes through
+// this function, and a debug line "sent: a001 LOGIN ..." is how a credential
+// ends up in a log file forever.
+func (c *Conn) exec(cmd string) ([]string, error) {
+	tag := c.nextTag()
+	if err := c.send(tag + " " + cmd); err != nil {
+		return nil, err
+	}
+	var lines []string
+	for {
+		line, err := c.readLine()
+		if err != nil {
+			return nil, err
+		}
+		if done, err := c.tagged(tag, line); done {
+			return lines, err
+		}
+		lines = append(lines, line)
+		// A response line may carry a literal (e.g. a header FETCH). Nothing we
+		// send asks for one outside Fetch, but skip it if it appears so the
+		// stream stays aligned.
+		if m := literalSize.FindStringSubmatch(strings.TrimSpace(line)); m != nil {
+			if n, err := strconv.Atoi(m[1]); err == nil {
+				if _, err := io.CopyN(io.Discard, c.r, int64(n)); err != nil {
+					return nil, err
+				}
+			}
+		}
+	}
+}
+
+// tagged reports whether line completes the command with this tag, and turns a
+// NO/BAD completion into an error. The error text is the server's, which never
+// echoes a password.
+func (c *Conn) tagged(tag, line string) (bool, error) {
+	if !strings.HasPrefix(line, tag+" ") {
+		return false, nil
+	}
+	rest := strings.TrimSpace(line[len(tag):])
+	switch {
+	case strings.HasPrefix(rest, "OK"):
+		return true, nil
+	case strings.HasPrefix(rest, "NO"), strings.HasPrefix(rest, "BAD"):
+		return true, fmt.Errorf("server said: %s", rest)
+	default:
+		return true, fmt.Errorf("unexpected completion: %s", rest)
+	}
+}
+
+func (c *Conn) send(line string) error {
+	c.setDeadline()
+	if _, err := io.WriteString(c.rwc, line+"\r\n"); err != nil {
+		return fmt.Errorf("email: write: %w", err)
+	}
+	return nil
+}
+
+func (c *Conn) readLine() (string, error) {
+	c.setDeadline()
+	line, err := c.r.ReadString('\n')
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimRight(line, "\r\n"), nil
+}
+
+// setDeadline applies the per-connection timeout when the transport supports
+// one. A hung IMAP server must not park the poller forever.
+func (c *Conn) setDeadline() {
+	if c.timeout <= 0 {
+		return
+	}
+	if d, ok := c.rwc.(interface{ SetDeadline(time.Time) error }); ok {
+		_ = d.SetDeadline(time.Now().Add(c.timeout))
+	}
+}
+
+// untagged splits "* SEARCH 1 2 3" into its payload when the key matches.
+func untagged(line, key string) (string, bool) {
+	if !strings.HasPrefix(line, "* ") {
+		return "", false
+	}
+	rest := strings.TrimSpace(line[2:])
+	if !strings.HasPrefix(rest, key) {
+		return "", false
+	}
+	return strings.TrimSpace(rest[len(key):]), true
+}
+
+// quote renders an IMAP quoted string. Passwords routinely contain characters
+// that would otherwise end the argument early, and CR/LF are stripped rather
+// than escaped because there is no legal way to send them — a credential file
+// with a stray newline must not become a second command.
+func quote(s string) string {
+	s = strings.NewReplacer("\r", "", "\n", "").Replace(s)
+	return `"` + strings.NewReplacer(`\`, `\\`, `"`, `\"`).Replace(s) + `"`
+}

internal/email/imap_test.go

@@ -0,0 +1,162 @@
+package email
+
+import (
+	"bufio"
+	"fmt"
+	"net"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+// fakeIMAP is a scripted server: enough of IMAP to exercise the client, and
+// nothing more. It records the commands it received so a test can assert on the
+// protocol (BODY.PEEK rather than BODY, EXAMINE rather than SELECT).
+type fakeIMAP struct {
+	msgs   map[uint32]string
+	uids   []uint32
+	cmds   []string
+	failOn string // substring of a command to answer NO
+}
+
+func (f *fakeIMAP) serve(t *testing.T, c net.Conn) {
+	t.Helper()
+	defer c.Close()
+	fmt.Fprint(c, "* OK fake IMAP ready\r\n")
+	r := bufio.NewReader(c)
+	for {
+		line, err := r.ReadString('\n')
+		if err != nil {
+			return
+		}
+		line = strings.TrimRight(line, "\r\n")
+		parts := strings.SplitN(line, " ", 2)
+		if len(parts) != 2 {
+			return
+		}
+		tag, cmd := parts[0], parts[1]
+		f.cmds = append(f.cmds, cmd)
+		if f.failOn != "" && strings.Contains(cmd, f.failOn) {
+			fmt.Fprintf(c, "%s NO computer says no\r\n", tag)
+			continue
+		}
+		upper := strings.ToUpper(cmd)
+		switch {
+		case strings.HasPrefix(upper, "LOGIN"), strings.HasPrefix(upper, "EXAMINE"):
+			fmt.Fprintf(c, "%s OK done\r\n", tag)
+		case strings.HasPrefix(upper, "UID SEARCH"):
+			var ids []string
+			for _, u := range f.uids {
+				ids = append(ids, strconv.FormatUint(uint64(u), 10))
+			}
+			fmt.Fprintf(c, "* SEARCH %s\r\n", strings.Join(ids, " "))
+			fmt.Fprintf(c, "%s OK search done\r\n", tag)
+		case strings.HasPrefix(upper, "UID FETCH"):
+			uid64, _ := strconv.ParseUint(strings.Fields(cmd)[2], 10, 32)
+			raw, ok := f.msgs[uint32(uid64)]
+			if ok {
+				fmt.Fprintf(c, "* 1 FETCH (UID %d BODY[] {%d}\r\n", uid64, len(raw))
+				fmt.Fprint(c, raw)
+				fmt.Fprint(c, ")\r\n")
+			}
+			fmt.Fprintf(c, "%s OK fetch done\r\n", tag)
+		case strings.HasPrefix(upper, "LOGOUT"):
+			fmt.Fprint(c, "* BYE\r\n")
+			fmt.Fprintf(c, "%s OK bye\r\n", tag)
+			return
+		default:
+			fmt.Fprintf(c, "%s BAD unknown\r\n", tag)
+		}
+	}
+}
+
+// dialFake wires a client Conn to an in-process server over net.Pipe.
+func dialFake(t *testing.T, f *fakeIMAP) *Conn {
+	t.Helper()
+	cli, srv := net.Pipe()
+	go f.serve(t, srv)
+	c, err := NewConn(cli, 5*time.Second)
+	if err != nil {
+		t.Fatalf("greeting: %v", err)
+	}
+	t.Cleanup(func() { c.Close() })
+	return c
+}
+
+func TestIMAPRoundTrip(t *testing.T) {
+	body := "Subject: hello\r\nContent-Type: text/plain; charset=utf-8\r\n\r\nCall the bank.\r\n"
+	f := &fakeIMAP{uids: []uint32{4, 9}, msgs: map[uint32]string{4: body, 9: body}}
+	c := dialFake(t, f)
+
+	if err := c.Login("kami", `pa"ss\word`); err != nil {
+		t.Fatalf("login: %v", err)
+	}
+	if err := c.Select("INBOX"); err != nil {
+		t.Fatalf("select: %v", err)
+	}
+	uids, err := c.SearchSince(time.Date(2026, 8, 1, 0, 0, 0, 0, time.UTC))
+	if err != nil {
+		t.Fatalf("search: %v", err)
+	}
+	if len(uids) != 2 || uids[0] != 4 || uids[1] != 9 {
+		t.Fatalf("uids = %v, want [4 9]", uids)
+	}
+	raw, err := c.Fetch(9)
+	if err != nil {
+		t.Fatalf("fetch: %v", err)
+	}
+	if string(raw) != body {
+		t.Errorf("fetched %q, want the literal verbatim", raw)
+	}
+	c.Logout()
+
+	joined := strings.Join(f.cmds, "\n")
+	// Read-only at the protocol level, and peeking — Maven must leave no trace
+	// of having read his mail.
+	if !strings.Contains(joined, "EXAMINE") || strings.Contains(joined, "SELECT ") {
+		t.Errorf("want EXAMINE (read-only), got:\n%s", joined)
+	}
+	if !strings.Contains(joined, "BODY.PEEK[]") {
+		t.Errorf("want BODY.PEEK, got:\n%s", joined)
+	}
+	// The password must have been quoted and escaped, not truncated at the quote.
+	if !strings.Contains(joined, `"pa\"ss\\word"`) {
+		t.Errorf("password not quoted correctly:\n%s", joined)
+	}
+	// SINCE must carry the IMAP date form.
+	if !strings.Contains(joined, "SINCE 1-Aug-2026") {
+		t.Errorf("want a SINCE date, got:\n%s", joined)
+	}
+}
+
+func TestIMAPServerNoIsAnError(t *testing.T) {
+	f := &fakeIMAP{failOn: "LOGIN"}
+	c := dialFake(t, f)
+	err := c.Login("kami", "wrong")
+	if err == nil {
+		t.Fatal("a NO completion must be an error")
+	}
+	// The error is the server's text; it must not echo the credential.
+	if strings.Contains(err.Error(), "wrong") {
+		t.Errorf("error leaks the password: %v", err)
+	}
+}
+
+func TestIMAPFetchMissingUID(t *testing.T) {
+	f := &fakeIMAP{uids: []uint32{1}, msgs: map[uint32]string{}}
+	c := dialFake(t, f)
+	raw, err := c.Fetch(1)
+	if err != nil {
+		t.Fatalf("fetch: %v", err)
+	}
+	if raw != nil {
+		t.Errorf("a vanished UID should give nil, got %q", raw)
+	}
+}
+
+func TestQuoteStripsNewlines(t *testing.T) {
+	if got := quote("pass\r\nA1 LOGOUT"); strings.ContainsAny(got, "\r\n") {
+		t.Errorf("quote kept a line break: %q", got)
+	}
+}

internal/email/junk.go

@@ -0,0 +1,80 @@
+package email
+
+import (
+	"net/mail"
+	"strings"
+)
+
+// The junk filter — the cheapest and most important half of reading mail.
+//
+// A mailbox is mostly machine-generated: newsletters, receipts nobody acts on,
+// social notifications, marketing. Sending all of it to a 1.7B and asking "is
+// there a task here" produces confident nonsense at a rate proportional to the
+// volume, so junk is decided by HEADERS, before any model sees the message.
+//
+// The rules are all bulk-mail markers that senders set on themselves, never
+// guesses about content:
+//
+//   - List-Unsubscribe / List-Id — by definition a mailing list. If he can
+//     unsubscribe from it, it is not asking him to do anything.
+//   - Precedence: bulk|junk|list — the sender declaring itself bulk.
+//   - Auto-Submitted other than "no" (RFC 3834) — generated by a machine.
+//   - X-Spam-Flag: YES, X-Spam-Status: Yes — the spam filter upstream already
+//     decided; we do not second-guess it in the other direction.
+//   - X-GM-LABELS / X-Gmail-Labels containing a Gmail category — Gmail's own
+//     Promotions/Social/Forums/Spam classification, when the server sends it.
+//
+// Deliberately NOT here: sender allow/deny lists and subject keyword matching.
+// Both are configuration that ages badly and both would be a place for his
+// contacts to end up in a config file. If a real correspondent's mail is being
+// dropped, the fix is a rule about a header, not a list of names.
+//
+// A junk verdict never deletes anything and never touches a flag on the server.
+// It means "do not spend the model on this", nothing more.
+
+// junkHeaders — headers whose mere presence marks bulk mail.
+var junkPresence = []string{"List-Unsubscribe", "List-Id", "List-Post"}
+
+// gmailCategories — Gmail's category labels, lowercased as they appear in
+// X-GM-LABELS. "important" and "inbox" are labels too, and are NOT categories.
+// Matching is by these exact tokens (substring is fine — they are namespaced
+// and cannot appear in a hand-made label by accident), so a user label named
+// "Social Club" is not mistaken for Gmail's Social category.
+var gmailCategories = []string{
+	"category_promotions", "category_social", "category_forums", "category_updates",
+	`\spam`, `\junk`,
+}
+
+// classifyJunk returns whether the message is bulk/automated and why. The
+// reason is a short header name, safe to log — it names the marker, never the
+// sender or the subject.
+func classifyJunk(h mail.Header) (bool, string) {
+	for _, name := range junkPresence {
+		if strings.TrimSpace(h.Get(name)) != "" {
+			return true, strings.ToLower(name)
+		}
+	}
+	switch strings.ToLower(strings.TrimSpace(h.Get("Precedence"))) {
+	case "bulk", "junk", "list":
+		return true, "precedence"
+	}
+	if v := strings.ToLower(strings.TrimSpace(h.Get("Auto-Submitted"))); v != "" && v != "no" {
+		return true, "auto-submitted"
+	}
+	if strings.EqualFold(strings.TrimSpace(h.Get("X-Spam-Flag")), "yes") {
+		return true, "x-spam-flag"
+	}
+	if v := strings.ToLower(strings.TrimSpace(h.Get("X-Spam-Status"))); strings.HasPrefix(v, "yes") {
+		return true, "x-spam-status"
+	}
+	labels := strings.ToLower(h.Get("X-GM-LABELS") + " " + h.Get("X-Gmail-Labels"))
+	for _, c := range gmailCategories {
+		if c == "" {
+			continue
+		}
+		if strings.Contains(labels, c) {
+			return true, "gmail-category"
+		}
+	}
+	return false, ""
+}

internal/email/junk_test.go

@@ -0,0 +1,59 @@
+package email
+
+import (
+	"net/mail"
+	"strings"
+	"testing"
+)
+
+func headers(t *testing.T, raw string) mail.Header {
+	t.Helper()
+	m, err := mail.ReadMessage(strings.NewReader(strings.ReplaceAll(raw, "\n", "\r\n") + "\r\n\r\nbody\r\n"))
+	if err != nil {
+		t.Fatalf("read headers: %v", err)
+	}
+	return m.Header
+}
+
+func TestClassifyJunk(t *testing.T) {
+	cases := []struct {
+		name   string
+		raw    string
+		junk   bool
+		reason string
+	}{
+		{"personal", "From: a@b.c\nSubject: привет", false, ""},
+		{"list-unsubscribe", "From: a@b.c\nList-Unsubscribe: <mailto:u@b.c>", true, "list-unsubscribe"},
+		{"list-id", "From: a@b.c\nList-Id: <golang-nuts.example>", true, "list-id"},
+		{"precedence bulk", "From: a@b.c\nPrecedence: bulk", true, "precedence"},
+		{"auto-submitted", "From: a@b.c\nAuto-Submitted: auto-generated", true, "auto-submitted"},
+		{"auto-submitted no", "From: a@b.c\nAuto-Submitted: no", false, ""},
+		{"spam flag", "From: a@b.c\nX-Spam-Flag: YES", true, "x-spam-flag"},
+		{"spam status", "From: a@b.c\nX-Spam-Status: Yes, score=9.1", true, "x-spam-status"},
+		{"spam status no", "From: a@b.c\nX-Spam-Status: No, score=0.1", false, ""},
+		{"gmail promo", "From: a@b.c\nX-Gmail-Labels: Inbox,CATEGORY_PROMOTIONS", true, "gmail-category"},
+		{"user label", "From: a@b.c\nX-Gmail-Labels: Social Club,Important", false, ""},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			junk, reason := classifyJunk(headers(t, c.raw))
+			if junk != c.junk || reason != c.reason {
+				t.Errorf("classifyJunk = (%v, %q), want (%v, %q)", junk, reason, c.junk, c.reason)
+			}
+		})
+	}
+}
+
+func TestNewsletterFixtureIsJunk(t *testing.T) {
+	msg, err := ParseMessage(9, fixture(t, "newsletter.eml"))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if !msg.Junk {
+		t.Fatal("a newsletter with List-Unsubscribe + Precedence: bulk must be junk")
+	}
+	// The reason is what gets logged, so it must never carry mail content.
+	if strings.Contains(msg.JunkReason, "@") || strings.Contains(msg.JunkReason, "Скидки") {
+		t.Errorf("junk reason leaks content: %q", msg.JunkReason)
+	}
+}

internal/email/message.go

@@ -0,0 +1,258 @@
+// Package email is the reading half of the email reader (Vikunja #246,
+// docs/plans/01-email-reader.md): a small IMAP client, a MIME-to-plaintext
+// converter, and the junk filter that decides a message is not worth reading at
+// all. Extraction lives in extract.go and writes nothing itself.
+//
+// Two constraints shape everything here, both from CLAUDE.md:
+//
+//   - Mail is personal. Nothing in this package logs a body, a subject, or an
+//     address; callers get the text and decide. Mail text is never search input
+//     — no function here reaches the network except the IMAP connection itself.
+//   - Off unless configured. There is no default host, no default account, and
+//     no fallback that would make a mailbox get read because a field was empty.
+//
+// The IMAP subset is deliberately tiny (LOGIN, SELECT, UID SEARCH, UID FETCH
+// with BODY.PEEK, LOGOUT). No IDLE: a poll every few minutes is what a task
+// candidate needs, and IDLE would mean holding a connection and a credential
+// open forever for latency nobody is waiting on.
+package email
+
+import (
+	"encoding/base64"
+	"fmt"
+	"io"
+	"mime"
+	"mime/multipart"
+	"mime/quotedprintable"
+	"net/mail"
+	"regexp"
+	"strings"
+)
+
+// MaxBodyBytes — how much of one message body is kept. A task hides in the
+// first screenful; the rest is signature, quoted history and legal boilerplate,
+// and it would only spend the resident model's 4096-token context.
+const MaxBodyBytes = 4000
+
+// Message — one mail, reduced to the fields extraction and review need.
+//
+// Raw is deliberately absent: once a message is parsed the original bytes are
+// dropped, so no caller can accidentally log or forward the whole mail.
+type Message struct {
+	UID     uint32
+	From    string
+	Subject string
+	Date    string // as sent, unparsed — display only
+	Body    string // plaintext, decoded, HTML-stripped, truncated
+	// Junk is set by the junk filter (see junk.go). A junk message is carried
+	// rather than dropped so the poller can count it and still mark it seen.
+	Junk       bool
+	JunkReason string
+}
+
+// ParseMessage turns one RFC 5322 message into a Message.
+//
+// It never fails on a body it cannot understand: an unparsable or
+// unsupported-charset body yields an empty Body and the headers still come
+// through, because a subject line alone is often the whole task ("Счёт за
+// интернет"). Only a message whose headers cannot be read at all is an error.
+func ParseMessage(uid uint32, raw []byte) (Message, error) {
+	m, err := mail.ReadMessage(strings.NewReader(string(raw)))
+	if err != nil {
+		return Message{}, fmt.Errorf("email: parse message: %w", err)
+	}
+	msg := Message{
+		UID:     uid,
+		From:    decodeHeader(m.Header.Get("From")),
+		Subject: decodeHeader(m.Header.Get("Subject")),
+		Date:    m.Header.Get("Date"),
+	}
+	msg.Junk, msg.JunkReason = classifyJunk(m.Header)
+	body, err := plaintextBody(m.Header.Get("Content-Type"), m.Header.Get("Content-Transfer-Encoding"), m.Body)
+	if err == nil {
+		msg.Body = truncate(collapse(body), MaxBodyBytes)
+	}
+	return msg, nil
+}
+
+// plaintextBody walks the MIME tree and returns the best plaintext it can.
+//
+// Preference order inside a multipart: text/plain first, text/html stripped
+// only when there is no plain part. multipart/mixed attachments are skipped
+// wholesale — an attachment is a file, not a sentence, and reading one would
+// mean parsing arbitrary formats from the network.
+func plaintextBody(contentType, encoding string, body io.Reader) (string, error) {
+	mediaType, params, err := mime.ParseMediaType(contentType)
+	if contentType == "" || err != nil {
+		// No Content-Type at all is legal and means text/plain; a broken one is
+		// treated the same rather than dropping the message.
+		mediaType, params = "text/plain", nil
+	}
+	switch {
+	case strings.HasPrefix(mediaType, "multipart/"):
+		boundary := params["boundary"]
+		if boundary == "" {
+			return "", fmt.Errorf("email: multipart without boundary")
+		}
+		return multipartText(multipart.NewReader(body, boundary))
+	case mediaType == "text/html":
+		raw, err := decodeBody(body, encoding, params["charset"])
+		if err != nil {
+			return "", err
+		}
+		return stripHTML(raw), nil
+	case mediaType == "text/plain":
+		return decodeBody(body, encoding, params["charset"])
+	default:
+		// A single-part non-text message (a bare PDF, say). No body, headers only.
+		return "", nil
+	}
+}
+
+// multipartText reads one multipart level, recursing into nested multiparts.
+// Returns the plain part if any part yielded one, else the stripped HTML.
+func multipartText(mr *multipart.Reader) (string, error) {
+	var plain, html string
+	for {
+		part, err := mr.NextPart()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			// A truncated multipart still gives up whatever came before it.
+			break
+		}
+		if part.FileName() != "" {
+			part.Close()
+			continue // attachment
+		}
+		ct := part.Header.Get("Content-Type")
+		mediaType, _, _ := mime.ParseMediaType(ct)
+		text, err := plaintextBody(ct, part.Header.Get("Content-Transfer-Encoding"), part)
+		part.Close()
+		if err != nil || strings.TrimSpace(text) == "" {
+			continue
+		}
+		if mediaType == "text/html" && !strings.HasPrefix(mediaType, "multipart/") {
+			if html == "" {
+				html = text
+			}
+			continue
+		}
+		if plain == "" {
+			plain = text
+		}
+	}
+	if strings.TrimSpace(plain) != "" {
+		return plain, nil
+	}
+	return html, nil
+}
+
+// decodeBody applies the transfer encoding, then the charset.
+//
+// Charset support is UTF-8 (and ASCII, its subset) only, on purpose: x/text's
+// encoding tables are not vendored here, and guessing at windows-1251 bytes
+// would feed the model mojibake it would happily extract a task from. An
+// unsupported charset returns an error, which ParseMessage turns into an empty
+// body — subject-only, which is honest.
+func decodeBody(r io.Reader, encoding, charset string) (string, error) {
+	switch strings.ToLower(strings.TrimSpace(encoding)) {
+	case "quoted-printable":
+		r = quotedprintable.NewReader(r)
+	case "base64":
+		r = newBase64Reader(r)
+	}
+	b, err := io.ReadAll(io.LimitReader(r, 1<<20))
+	if err != nil && len(b) == 0 {
+		return "", fmt.Errorf("email: read body: %w", err)
+	}
+	switch cs := strings.ToLower(strings.TrimSpace(charset)); cs {
+	case "", "utf-8", "utf8", "us-ascii", "ascii":
+		return string(b), nil
+	default:
+		return "", fmt.Errorf("email: unsupported charset %q", cs)
+	}
+}
+
+// decodeHeader decodes RFC 2047 encoded words ("=?utf-8?B?...?="), which is how
+// every Russian subject line arrives. Undecodable headers come back as-is
+// rather than empty: a mangled subject is still a hint, and it is only ever
+// shown to him as evidence.
+func decodeHeader(v string) string {
+	dec := new(mime.WordDecoder)
+	out, err := dec.DecodeHeader(v)
+	if err != nil {
+		return collapse(v)
+	}
+	return collapse(out)
+}
+
+var (
+	scriptStyle = regexp.MustCompile(`(?is)<(script|style)\b[^>]*>.*?</\s*(script|style)\s*>`)
+	htmlBreak   = regexp.MustCompile(`(?i)<\s*(br\s*/?|/p|/div|/tr|/li|/h[1-6])\s*>`)
+	htmlTag     = regexp.MustCompile(`(?s)<[^>]*>`)
+	htmlComment = regexp.MustCompile(`(?s)<!--.*?-->`)
+)
+
+// stripHTML reduces an HTML part to text. A regex stripper, not a parser:
+// x/net/html is not vendored, and the consumer is a model reading prose — a
+// stray angle bracket costs nothing, whereas a new dependency for the privacy-
+// sensitive path costs review.
+func stripHTML(s string) string {
+	s = scriptStyle.ReplaceAllString(s, " ")
+	s = htmlComment.ReplaceAllString(s, " ")
+	s = htmlBreak.ReplaceAllString(s, "\n")
+	s = htmlTag.ReplaceAllString(s, " ")
+	return unescapeEntities(s)
+}
+
+var entities = strings.NewReplacer(
+	"&nbsp;", " ", "&amp;", "&", "&lt;", "<", "&gt;", ">",
+	"&quot;", `"`, "&#39;", "'", "&apos;", "'", "&mdash;", "—", "&ndash;", "–",
+)
+
+func unescapeEntities(s string) string { return entities.Replace(s) }
+
+// collapse squeezes runs of whitespace, keeping single newlines. Mail bodies
+// arrive with hard-wrapped lines and blocks of blank space; the model does not
+// need them and they are pure context budget.
+func collapse(s string) string {
+	lines := strings.Split(strings.ReplaceAll(s, "\r\n", "\n"), "\n")
+	var out []string
+	blank := 0
+	for _, l := range lines {
+		l = strings.TrimSpace(strings.Join(strings.Fields(l), " "))
+		if l == "" {
+			blank++
+			if blank > 1 {
+				continue
+			}
+			out = append(out, "")
+			continue
+		}
+		blank = 0
+		out = append(out, l)
+	}
+	return strings.TrimSpace(strings.Join(out, "\n"))
+}
+
+// truncate cuts to n bytes on a rune boundary.
+func truncate(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	cut := s[:n]
+	for len(cut) > 0 && !isRuneStart(cut[len(cut)-1]) {
+		cut = cut[:len(cut)-1]
+	}
+	return strings.TrimSpace(cut) + "…"
+}
+
+func isRuneStart(b byte) bool { return b&0xC0 != 0x80 }
+
+// newBase64Reader — base64.NewDecoder already skips the CRLFs mail bodies wrap
+// with, so this is only a named seam for decodeBody to read cleanly.
+func newBase64Reader(r io.Reader) io.Reader {
+	return base64.NewDecoder(base64.StdEncoding, r)
+}

internal/email/message_test.go

@@ -0,0 +1,111 @@
+package email
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func fixture(t *testing.T, name string) []byte {
+	t.Helper()
+	b, err := os.ReadFile(filepath.Join("testdata", name))
+	if err != nil {
+		t.Fatalf("read fixture %s: %v", name, err)
+	}
+	return b
+}
+
+func TestParsePlainRussian(t *testing.T) {
+	msg, err := ParseMessage(7, fixture(t, "plain_ru.eml"))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if msg.UID != 7 {
+		t.Errorf("uid = %d, want 7", msg.UID)
+	}
+	if want := "Нужно закрыть задачу"; msg.Subject != want {
+		t.Errorf("subject = %q, want %q", msg.Subject, want)
+	}
+	if !strings.Contains(msg.From, "Антон") {
+		t.Errorf("from = %q, want the decoded display name", msg.From)
+	}
+	if !strings.Contains(msg.Body, "Надо отправить акт до пятницы.") {
+		t.Errorf("body = %q, want the quoted-printable text decoded", msg.Body)
+	}
+	if msg.Junk {
+		t.Errorf("a personal mail must not be junk (%s)", msg.JunkReason)
+	}
+}
+
+func TestParseHTMLOnlyIsStripped(t *testing.T) {
+	msg, err := ParseMessage(1, fixture(t, "html_only.eml"))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if strings.Contains(msg.Body, "<") || strings.Contains(msg.Body, "color:red") || strings.Contains(msg.Body, "x()") {
+		t.Errorf("body still has markup/script/style: %q", msg.Body)
+	}
+	for _, want := range []string{"Счёт за интернет: 700", "Оплатить до 5 августа."} {
+		if !strings.Contains(msg.Body, want) {
+			t.Errorf("body = %q, want it to contain %q", msg.Body, want)
+		}
+	}
+	// &nbsp; must have become a real space, not vanished into the number.
+	if strings.Contains(msg.Body, "&nbsp;") {
+		t.Errorf("entity left unescaped: %q", msg.Body)
+	}
+}
+
+func TestParsePrefersPlainAndSkipsAttachments(t *testing.T) {
+	msg, err := ParseMessage(2, fixture(t, "mixed_attachment.eml"))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if got := strings.TrimSpace(msg.Body); got != "Sign the contract before Monday." {
+		t.Errorf("body = %q, want the text/plain alternative only", got)
+	}
+	if strings.Contains(msg.Body, "PDF") {
+		t.Errorf("attachment bytes leaked into the body: %q", msg.Body)
+	}
+}
+
+// An unsupported charset must degrade to headers-only rather than to mojibake
+// the model would then extract a task from.
+func TestParseUnsupportedCharsetKeepsHeaders(t *testing.T) {
+	msg, err := ParseMessage(3, fixture(t, "cp1251.eml"))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if msg.Subject != "Legacy" {
+		t.Errorf("subject = %q, want Legacy", msg.Subject)
+	}
+	if msg.Body != "" {
+		t.Errorf("body = %q, want empty for an undecodable charset", msg.Body)
+	}
+}
+
+func TestParseTruncatesLongBody(t *testing.T) {
+	var b strings.Builder
+	b.WriteString("Subject: long\r\nContent-Type: text/plain; charset=utf-8\r\n\r\n")
+	for i := 0; i < 2000; i++ {
+		b.WriteString("длинная строка ")
+	}
+	msg, err := ParseMessage(4, []byte(b.String()))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if len(msg.Body) > MaxBodyBytes+8 {
+		t.Errorf("body kept %d bytes, want ≤ %d", len(msg.Body), MaxBodyBytes)
+	}
+	if !strings.HasSuffix(msg.Body, "…") {
+		t.Errorf("truncated body should be marked: %q", msg.Body[len(msg.Body)-20:])
+	}
+}
+
+func TestCollapseSqueezesBlankLines(t *testing.T) {
+	got := collapse("  a  b \r\n\r\n\r\n\r\n  c  \r\n")
+	if got != "a b\n\nc" {
+		t.Errorf("collapse = %q, want %q", got, "a b\n\nc")
+	}
+}

internal/email/testdata/cp1251.eml

@@ -0,0 +1,7 @@
+From: legacy@example.org
+To: kami@example.org
+Subject: Legacy
+Date: Fri, 01 Aug 2026 05:00:00 +0400
+Content-Type: text/plain; charset="windows-1251"
+
+Ï

internal/email/testdata/html_only.eml

@@ -0,0 +1,13 @@
+From: billing@isp.example
+To: kami@example.org
+Subject: =?utf-8?B?0KHRh9GR0YIg0LfQsCDQuNC90YLQtdGA0L3QtdGC?=
+Date: Fri, 01 Aug 2026 08:00:00 +0400
+MIME-Version: 1.0
+Content-Type: multipart/alternative; boundary="B1"
+
+--B1
+Content-Type: text/html; charset="utf-8"
+Content-Transfer-Encoding: base64
+
+PGh0bWw+PGhlYWQ+PHN0eWxlPnB7Y29sb3I6cmVkfTwvc3R5bGU+PC9oZWFkPjxib2R5PjxwPtCh0YfRkdGCINC30LAg0LjQvdGC0LXRgNC90LXRgjogNzAwJm5ic3A74oK9PC9wPjxwPtCe0L/Qu9Cw0YLQuNGC0Ywg0LTQviA1INCw0LLQs9GD0YHRgtCwLjwvcD48c2NyaXB0PngoKTwvc2NyaXB0PjwvYm9keT48L2h0bWw+
+--B1--

internal/email/testdata/mixed_attachment.eml

@@ -0,0 +1,26 @@
+From: hr@work.example
+To: kami@example.org
+Subject: Contract
+Date: Fri, 01 Aug 2026 07:00:00 +0400
+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary="M1"
+
+--M1
+Content-Type: multipart/alternative; boundary="A1"
+
+--A1
+Content-Type: text/plain; charset="utf-8"
+
+Sign the contract before Monday.
+--A1
+Content-Type: text/html; charset="utf-8"
+
+<p>Sign the contract before Monday.</p>
+--A1--
+--M1
+Content-Type: application/pdf; name="contract.pdf"
+Content-Disposition: attachment; filename="contract.pdf"
+Content-Transfer-Encoding: base64
+
+JVBERi0xLjQgbm90IHJlYWxseSBhIHBkZg==
+--M1--

internal/email/testdata/newsletter.eml

@@ -0,0 +1,9 @@
+From: news@shop.example
+To: kami@example.org
+Subject: =?utf-8?B?0KHQutC40LTQutC4INGC0L7Qu9GM0LrQviDRgdC10LPQvtC00L3Rjw==?=
+Date: Fri, 01 Aug 2026 06:00:00 +0400
+List-Unsubscribe: <mailto:unsub@shop.example>
+Precedence: bulk
+Content-Type: text/plain; charset="utf-8"
+
+Sale!

internal/email/testdata/plain_ru.eml

@@ -0,0 +1,14 @@
+From: =?utf-8?B?0JDQvdGC0L7QvQ==?= <anton@example.org>
+To: kami@example.org
+Subject: =?utf-8?B?0J3Rg9C20L3QviDQt9Cw0LrRgNGL0YLRjCDQt9Cw0LTQsNGH0YM=?=
+Date: Fri, 01 Aug 2026 09:12:00 +0400
+Content-Type: text/plain; charset="utf-8"
+Content-Transfer-Encoding: quoted-printable
+Message-ID: <plain-ru@example.org>
+
+=D0=9F=D1=80=D0=B8=D0=B2=D0=B5=D1=82! =D0=9D=D0=B0=D0=B4=D0=BE =D0=BE=D1=82=
+=D0=BF=D1=80=D0=B0=D0=B2=D0=B8=D1=82=D1=8C =D0=B0=D0=BA=D1=82 =D0=B4=D0=BE =
+=D0=BF=D1=8F=D1=82=D0=BD=D0=B8=D1=86=D1=8B.
+
+--
+Anton