queue: deliver to multiple recipients in a single smtp transaction

transferring the data only once. we only do this when the recipient domains
are the same. when queuing, we now take care to set the same NextAttempt
timestamp, so queued messages are actually eligable for combined delivery.

this adds a DeliverMultiple to the smtp client. for pipelined requests, it will
send all RCPT TO (and MAIL and DATA) in one go, and handles the various
responses and error conditions, returning either an overal error, or per
recipient smtp responses. the results of the smtp LIMITS extension are also
available in the smtp client now.

this also takes the "LIMITS RCPTMAX" smtp extension into account: if the server
only accepts a single recipient, we won't send multiple.
if a server doesn't announce a RCPTMAX limit, but still has one (like mox does
for non-spf-verified transactions), we'll recognize code 452 and 552 (for
historic reasons) as temporary error, and try again in a separate transaction
immediately after. we don't yet implement "LIMITS MAILMAX", doesn't seem likely
in practice.

queue/dsn.go

@@ -3,8 +3,10 @@ package queue
 import (
 	"bufio"
 	"context"
+	"errors"
 	"fmt"
 	"log/slog"
+	"net"
 	"os"
 	"strings"
 	"time"
@@ -12,12 +14,15 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 
+	"github.com/mjl-/bstore"
+
 	"github.com/mjl-/mox/dns"
 	"github.com/mjl-/mox/dsn"
 	"github.com/mjl-/mox/message"
 	"github.com/mjl-/mox/mlog"
 	"github.com/mjl-/mox/mox-"
 	"github.com/mjl-/mox/smtp"
+	"github.com/mjl-/mox/smtpclient"
 	"github.com/mjl-/mox/store"
 )
 
@@ -30,6 +35,79 @@ var (
 	)
 )
 
+// todo: rename function, perhaps put some of the params in a delivery struct so we don't pass all the params all the time?
+func fail(ctx context.Context, qlog mlog.Log, msgs []*Msg, dialedIPs map[string][]net.IP, backoff time.Duration, remoteMTA dsn.NameIP, err error) {
+	// todo future: when we implement relaying, we should be able to send DSNs to non-local users. and possibly specify a null mailfrom. ../rfc/5321:1503
+	// todo future: when we implement relaying, and a dsn cannot be delivered, and requiretls was active, we cannot drop the message. instead deliver to local postmaster? though ../rfc/8689:383 may intend to say the dsn should be delivered without requiretls?
+	// todo future: when we implement smtp dsn extension, parameter RET=FULL must be disregarded for messages with REQUIRETLS. ../rfc/8689:379
+
+	m0 := msgs[0]
+
+	var smtpLines []string
+	var cerr smtpclient.Error
+	var permanent bool
+	var errmsg = err.Error()
+	var code int
+	var secodeOpt string
+	if errors.As(err, &cerr) {
+		if cerr.Line != "" {
+			smtpLines = append([]string{cerr.Line}, cerr.MoreLines...)
+		}
+		permanent = cerr.Permanent
+		code = cerr.Code
+		secodeOpt = cerr.Secode
+	}
+	qlog = qlog.With(
+		slog.Bool("permanent", permanent),
+		slog.Int("code", code),
+		slog.String("secode", secodeOpt),
+	)
+
+	ids := make([]int64, len(msgs))
+	for i, m := range msgs {
+		ids[i] = m.ID
+	}
+
+	if permanent || m0.MaxAttempts == 0 && m0.Attempts >= 8 || m0.MaxAttempts > 0 && m0.Attempts >= m0.MaxAttempts {
+		for _, m := range msgs {
+			qmlog := qlog.With(slog.Int64("msgid", m.ID), slog.Any("recipient", m.Recipient()))
+			qmlog.Errorx("permanent failure delivering from queue", err)
+			deliverDSNFailure(ctx, qmlog, *m, remoteMTA, secodeOpt, errmsg, smtpLines)
+		}
+		if err := queueDelete(context.Background(), ids...); err != nil {
+			qlog.Errorx("deleting messages from queue after permanent failure", err)
+		}
+		return
+	}
+
+	// All messages should have the same DialedIPs, so we can update them all at once.
+	qup := bstore.QueryDB[Msg](context.Background(), DB)
+	qup.FilterIDs(ids)
+	if _, xerr := qup.UpdateNonzero(Msg{LastError: errmsg, DialedIPs: dialedIPs}); err != nil {
+		qlog.Errorx("storing delivery error", xerr, slog.String("deliveryerror", errmsg))
+	}
+
+	if m0.Attempts == 5 {
+		// We've attempted deliveries at these intervals: 0, 7.5m, 15m, 30m, 1h, 2u.
+		// Let sender know delivery is delayed.
+
+		retryUntil := m0.LastAttempt.Add((4 + 8 + 16) * time.Hour)
+		for _, m := range msgs {
+			qmlog := qlog.With(slog.Int64("msgid", m.ID), slog.Any("recipient", m.Recipient()))
+			qmlog.Errorx("temporary failure delivering from queue, sending delayed dsn", err, slog.Duration("backoff", backoff))
+			deliverDSNDelay(ctx, qmlog, *m, remoteMTA, secodeOpt, errmsg, smtpLines, retryUntil)
+		}
+	} else {
+		for _, m := range msgs {
+			qlog.Errorx("temporary failure delivering from queue", err,
+				slog.Int64("msgid", m.ID),
+				slog.Any("recipient", m.Recipient()),
+				slog.Duration("backoff", backoff),
+				slog.Time("nextattempt", m0.NextAttempt))
+		}
+	}
+}
+
 func deliverDSNFailure(ctx context.Context, log mlog.Log, m Msg, remoteMTA dsn.NameIP, secodeOpt, errmsg string, smtpLines []string) {
 	const subject = "mail delivery failed"
 	message := fmt.Sprintf(`