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.

smtpserver/dsn.go

@@ -3,6 +3,7 @@ package smtpserver
 import (
 	"context"
 	"fmt"
+	"time"
 
 	"github.com/mjl-/mox/dsn"
 	"github.com/mjl-/mox/mlog"
@@ -53,7 +54,7 @@ func queueDSN(ctx context.Context, log mlog.Log, c *conn, rcptTo smtp.Path, m ds
 	if requireTLS {
 		reqTLS = &requireTLS
 	}
-	qm := queue.MakeMsg(smtp.Path{}, rcptTo, has8bit, smtputf8, int64(len(buf)), m.MessageID, nil, reqTLS)
+	qm := queue.MakeMsg(smtp.Path{}, rcptTo, has8bit, smtputf8, int64(len(buf)), m.MessageID, nil, reqTLS, time.Now())
 	qm.DSNUTF8 = bufUTF8
 	if err := queue.Add(ctx, c.log, "", f, qm); err != nil {
 		return err

smtpserver/server.go

@@ -1834,7 +1834,13 @@ func (c *conn) cmdData(p *parser) {
 			tlsComment := mox.TLSReceivedComment(c.log, tlsConn.ConnectionState())
 			recvHdr.Add(" ", tlsComment...)
 		}
-		recvHdr.Add(" ", "for", "<"+rcptTo+">;", time.Now().Format(message.RFC5322Z))
+		// We leave out an empty "for" clause. This is empty for messages submitted to
+		// multiple recipients, so the message stays identical and a single smtp
+		// transaction can deliver, only transferring the data once.
+		if rcptTo != "" {
+			recvHdr.Add(" ", "for", "<"+rcptTo+">;")
+		}
+		recvHdr.Add(" ", time.Now().Format(message.RFC5322Z))
 		return recvHdr.String()
 	}
 
@@ -1979,6 +1985,7 @@ func (c *conn) submit(ctx context.Context, recvHdrFor func(string) string, msgWr
 	// We always deliver through the queue. It would be more efficient to deliver
 	// directly, but we don't want to circumvent all the anti-spam measures. Accounts
 	// on a single mox instance should be allowed to block each other.
+	now := time.Now()
 	qml := make([]queue.Msg, len(c.recipients))
 	for i, rcptAcc := range c.recipients {
 		if Localserve {
@@ -1993,9 +2000,16 @@ func (c *conn) submit(ctx context.Context, recvHdrFor func(string) string, msgWr
 			}
 		}
 
-		xmsgPrefix := append([]byte(recvHdrFor(rcptAcc.rcptTo.String())), msgPrefix...)
+		// For multiple recipients, we don't make each message prefix unique, leaving out
+		// the "for" clause in the Received header. This allows the queue to deliver the
+		// messages in a single smtp transaction.
+		var rcptTo string
+		if len(c.recipients) == 1 {
+			rcptTo = rcptAcc.rcptTo.String()
+		}
+		xmsgPrefix := append([]byte(recvHdrFor(rcptTo)), msgPrefix...)
 		msgSize := int64(len(xmsgPrefix)) + msgWriter.Size
-		qm := queue.MakeMsg(*c.mailFrom, rcptAcc.rcptTo, msgWriter.Has8bit, c.smtputf8, msgSize, messageID, xmsgPrefix, c.requireTLS)
+		qm := queue.MakeMsg(*c.mailFrom, rcptAcc.rcptTo, msgWriter.Has8bit, c.smtputf8, msgSize, messageID, xmsgPrefix, c.requireTLS, now)
 		if !c.futureRelease.IsZero() {
 			qm.NextAttempt = c.futureRelease
 			qm.FutureReleaseRequest = c.futureReleaseRequest