mox/tlsrptsend/send.go

// Package tlsrptsend sends TLS reports based on success/failure statistics and
// details gathering while making SMTP STARTTLS connections for delivery. See RFC
// 8460.
package tlsrptsend

// tlsrptsend is a separate package instead of being in tlsrptdb because it imports
// queue and queue imports tlsrptdb to store tls results, so that would cause a
// cyclic dependency.

// Sending TLS reports and DMARC reports is very similar. See ../dmarcdb/eval.go:/similar and ../tlsrptsend/send.go:/similar.

// todo spec: ../rfc/8460:441 ../rfc/8460:463 may lead reader to believe they can find a DANE or MTA-STS policy at the same place, while in practice you'll get an MTA-STS policy at a recipient domain and a DANE policy at a mail host, and that's where the TLSRPT policy is defined. it would have helped with this implementation if the distinction was mentioned explicitly, also earlier in the document (i realized it late in the implementation process based on the terminology entry for the policy domain). examples with a tlsrpt record at a mail host would have helped too.
// todo spec: ../rfc/8460:1017 example report message misses the required DKIM signature.

import (
	"compress/gzip"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"net/textproto"
	"net/url"
	"os"
	"runtime/debug"
	"strings"
	"sync"
	"time"

	"golang.org/x/exp/slices"
	"golang.org/x/exp/slog"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"

	"github.com/mjl-/bstore"

	"github.com/mjl-/mox/config"
	"github.com/mjl-/mox/dkim"
	"github.com/mjl-/mox/dns"
	"github.com/mjl-/mox/message"
	"github.com/mjl-/mox/metrics"
	"github.com/mjl-/mox/mlog"
	"github.com/mjl-/mox/mox-"
	"github.com/mjl-/mox/moxio"
	"github.com/mjl-/mox/moxvar"
	"github.com/mjl-/mox/queue"
	"github.com/mjl-/mox/smtp"
	"github.com/mjl-/mox/store"
	"github.com/mjl-/mox/tlsrpt"
	"github.com/mjl-/mox/tlsrptdb"
)

var (
	metricReport = promauto.NewCounter(
		prometheus.CounterOpts{
			Name: "mox_tlsrptsend_report_queued_total",
			Help: "Total messages with TLS reports queued.",
		},
	)
	metricReportError = promauto.NewCounter(
		prometheus.CounterOpts{
			Name: "mox_tlsrptsend_report_error_total",
			Help: "Total errors while composing or queueing TLS reports.",
		},
	)
)

var jitterRand = mox.NewPseudoRand()

// time to sleep until sending reports at midnight t, replaced by tests.
// Jitter so we don't cause load at exactly midnight, other processes may
// already be doing that.
var jitteredTimeUntil = func(t time.Time) time.Duration {
	return time.Until(t.Add(time.Duration(240+jitterRand.Intn(120)) * time.Second))
}

// Start launches a goroutine that wakes up just after 00:00 UTC to send TLSRPT
// reports. Reports are sent spread out over a 4 hour period.
func Start(resolver dns.Resolver) {
	go func() {
		log := mlog.New("tlsrptsend", nil)

		defer func() {
			// In case of panic don't take the whole program down.
			x := recover()
			if x != nil {
				log.Error("recover from panic", slog.Any("panic", x))
				debug.PrintStack()
				metrics.PanicInc(metrics.Tlsrptdb)
			}
		}()

		timer := time.NewTimer(time.Hour) // Reset below.
		defer timer.Stop()

		ctx := mox.Shutdown

		db := tlsrptdb.ResultDB
		if db == nil {
			log.Error("no tlsrpt results database for tls reports, not sending reports")
			return
		}

		// We start sending for previous day, if there are any reports left.
		endUTC := midnightUTC(time.Now())

		for {
			dayUTC := endUTC.Add(-12 * time.Hour).Format("20060102")

			// Remove evaluations older than 48 hours (2 reports with 24 hour interval)
			// They should have been processed by now. We may have kept them
			// during temporary errors, but persistent temporary errors shouldn't fill up our
			// database and we don't want to send old reports either.
			_, err := bstore.QueryDB[tlsrptdb.TLSResult](ctx, db).FilterLess("DayUTC", endUTC.Add((-48-12)*time.Hour).Format("20060102")).Delete()
			log.Check(err, "removing stale tls results from database")

			clog := log.WithCid(mox.Cid())
			clog.Info("sending tls reports", slog.String("day", dayUTC))
			if err := sendReports(ctx, clog, resolver, db, dayUTC, endUTC); err != nil {
				clog.Errorx("sending tls reports", err)
				metricReportError.Inc()
			} else {
				clog.Info("finished sending tls reports")
			}

			endUTC = endUTC.Add(24 * time.Hour)
			timer.Reset(jitteredTimeUntil(endUTC))

			select {
			case <-ctx.Done():
				log.Info("tls report sender shutting down")
				return
			case <-timer.C:
			}
		}
	}()
}

func midnightUTC(now time.Time) time.Time {
	t := now.UTC()
	return time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, t.Location())
}

// Sleep in between sending two reports.
// Replaced by tests.
var sleepBetween = func(ctx context.Context, between time.Duration) (ok bool) {
	t := time.NewTimer(between)
	select {
	case <-ctx.Done():
		t.Stop()
		return false
	case <-t.C:
		return true
	}
}

// sendReports gathers all policy domains that have results that should receive a
// TLS report and sends a report to each if their TLSRPT DNS record has reporting
// addresses.
func sendReports(ctx context.Context, log mlog.Log, resolver dns.Resolver, db *bstore.DB, dayUTC string, endTimeUTC time.Time) error {
	type key struct {
		policyDomain string
		dayUTC       string
	}

	// Gather all policy domains we plan to send to.
	rcptDoms := map[key]bool{}    // Results where recipient domain is equal to policy domain, regardless of IsHost.
	nonRcptDoms := map[key]bool{} // MX domains (without those that are also recipient domains).
	var nsend int
	q := bstore.QueryDB[tlsrptdb.TLSResult](ctx, db)
	q.FilterLessEqual("DayUTC", dayUTC)
	err := q.ForEach(func(e tlsrptdb.TLSResult) error {
		doms := rcptDoms
		if e.PolicyDomain != e.RecipientDomain {
			doms = nonRcptDoms
		}
		k := key{e.PolicyDomain, e.DayUTC}
		if e.SendReport && !doms[k] {
			nsend++
		}
		doms[k] = doms[k] || e.SendReport
		return nil
	})
	if err != nil {
		return fmt.Errorf("looking for domains to send tls reports to: %v", err)
	}

	// Stretch sending reports over max 4 hours, but only if there are quite a few
	// messages. ../rfc/8460:479
	between := 4 * time.Hour
	if nsend > 0 {
		between = between / time.Duration(nsend)
	}
	if between > 5*time.Minute {
		between = 5 * time.Minute
	}

	var wg sync.WaitGroup

	var n int

	remove := map[key]struct{}{}
	var removeMutex sync.Mutex

	sendDomains := func(isRcptDom bool, doms map[key]bool) {
		for k, send := range doms {
			if !send {
				removeMutex.Lock()
				remove[k] = struct{}{}
				removeMutex.Unlock()
				continue
			}

			if n > 0 {
				ok := sleepBetween(ctx, between)
				if !ok {
					return
				}
			}
			n++

			// In goroutine, so our timing stays independent of how fast we process.
			wg.Add(1)
			go func(k key) {
				defer func() {
					// In case of panic don't take the whole program down.
					x := recover()
					if x != nil {
						log.Error("unhandled panic in tlsrptsend sendReports", slog.Any("panic", x))
						debug.PrintStack()
						metrics.PanicInc(metrics.Tlsrptdb)
					}
				}()
				defer wg.Done()

				rlog := log.WithCid(mox.Cid()).With(slog.String("policydomain", k.policyDomain),
					slog.String("daytutc", k.dayUTC),
					slog.Bool("isrcptdom", isRcptDom))
				rlog.Info("looking to send tls report for domain")
				cleanup, err := sendReportDomain(ctx, rlog, resolver, db, endTimeUTC, isRcptDom, k.policyDomain, k.dayUTC)
				if err != nil {
					rlog.Errorx("sending tls report to domain", err)
					metricReportError.Inc()
				}
				if cleanup {
					removeMutex.Lock()
					defer removeMutex.Unlock()
					remove[k] = struct{}{}
				}
			}(k)
		}
	}

	// We send to recipient domains first. That will store the reporting addresses for
	// the recipient domains, which are used when sending to nonRcptDoms to potentially
	// skip sending a duplicate report.
	sendDomains(true, rcptDoms)
	wg.Wait()
	sendDomains(false, nonRcptDoms)
	wg.Wait()

	// Remove all records that have been processed.
	err = db.Write(ctx, func(tx *bstore.Tx) error {
		for k := range remove {
			q := bstore.QueryTx[tlsrptdb.TLSResult](tx)
			q.FilterNonzero(tlsrptdb.TLSResult{PolicyDomain: k.policyDomain, DayUTC: k.dayUTC})
			_, err := q.Delete()
			if err != nil {
				return err
			}
		}
		return nil
	})
	log.Check(err, "cleaning up tls results in database")

	return nil
}

// replaceable for testing.
var queueAdd = queue.Add

func sendReportDomain(ctx context.Context, log mlog.Log, resolver dns.Resolver, db *bstore.DB, endUTC time.Time, isRcptDom bool, policyDomain, dayUTC string) (cleanup bool, rerr error) {
	polDom, err := dns.ParseDomain(policyDomain)
	if err != nil {
		return false, fmt.Errorf("parsing policy domain for sending tls reports: %v", err)
	}

	// Reports need to be DKIM-signed by the submitter domain. Lookup the DKIM
	// configuration now. If we don't have any, there is no point sending reports.
	// todo spec: ../rfc/8460:322 "reporting domain" is a bit ambiguous. submitter domain is used in other places. it may be helpful in practice to allow dmarc-relaxed-like matching of the signing domain, so an address postmaster at mail host can send the reports using dkim keys at a higher-up domain (e.g. the publicsuffix domain).
	fromDom := mox.Conf.Static.HostnameDomain
	var confDKIM config.DKIM
	for {
		confDom, ok := mox.Conf.Domain(fromDom)
		if len(confDom.DKIM.Sign) > 0 {
			confDKIM = confDom.DKIM
			break
		} else if ok {
			return true, fmt.Errorf("domain for mail host does not have dkim signing configured, report message cannot be dkim-signed")
		}

		// Remove least significant label.
		var nfd dns.Domain
		_, nfd.ASCII, _ = strings.Cut(fromDom.ASCII, ".")
		_, nfd.Unicode, _ = strings.Cut(fromDom.Unicode, ".")
		fromDom = nfd

		var zerodom dns.Domain
		if fromDom == zerodom {
			return true, fmt.Errorf("no configured domain for mail host found, report message cannot be dkim-signed")
		}
	}

	// We'll cleanup records by default.
	cleanup = true
	// But if we encounter a temporary error we cancel cleanup of evaluations on error.
	tempError := false

	defer func() {
		if !cleanup || tempError {
			cleanup = false
			log.Debug("not cleaning up results after attempting to send tls report")
		}
	}()

	// Get TLSRPT record. If there are no reporting addresses, we're not going to send at all.
	record, _, err := tlsrpt.Lookup(ctx, log.Logger, resolver, polDom)
	if err != nil {
		// If there is no TLSRPT record, that's fine, we'll remove what we tracked.
		if errors.Is(err, tlsrpt.ErrNoRecord) {
			return true, nil
		}
		cleanup = errors.Is(err, tlsrpt.ErrDNS)
		return cleanup, fmt.Errorf("looking up current tlsrpt record for reporting addresses: %v", err)
	}

	var recipients []message.NameAddress
	var recipientStrs []string

	for _, l := range record.RUAs {
		for _, s := range l {
			u, err := url.Parse(string(s))
			if err != nil {
				log.Debugx("parsing rua uri in tlsrpt dns record, ignoring", err, slog.Any("rua", s))
				continue
			}

			if u.Scheme == "mailto" {
				addr, err := smtp.ParseAddress(u.Opaque)
				if err != nil {
					log.Debugx("parsing mailto uri in tlsrpt record rua value, ignoring", err, slog.Any("rua", s))
					continue
				}
				recipients = append(recipients, message.NameAddress{Address: addr})
				recipientStrs = append(recipientStrs, string(s))
			} else if u.Scheme == "https" {
				// Although "report" is ambiguous and could mean both only the JSON data or an
				// entire message (including DKIM-Signature) with the JSON data, it appears the
				// intention of the RFC is that the HTTPS transport sends only the JSON data, given
				// mention of the media type to use (for the HTTP POST). It is the type of the
				// report, not of a message. TLS reports sent over email must have a DKIM
				// signature, i.e. must be authenticated, for understandable reasons. No such
				// requirement is specified for HTTPS, but no one is going to accept
				// unauthenticated TLS reports over HTTPS. So there seems little point in sending
				// them.
				// ../rfc/8460:320 ../rfc/8460:1055
				// todo spec: would be good to have clearer distinction between "report" (JSON) and "report message" (message with report attachment, that can be DKIM signed). propose sending report message over https that includes DKIM signature so authenticity can be verified and the report used. ../rfc/8460:310
				log.Debug("https scheme in rua uri in tlsrpt record, ignoring since they will likey not be used to due lack of authentication", slog.Any("rua", s))
			} else {
				log.Debug("unknown scheme in rua uri in tlsrpt record, ignoring", slog.Any("rua", s))
			}
		}
	}

	if len(recipients) == 0 {
		// No reports requested, perfectly fine, no work to do for us.
		log.Debug("no tlsrpt reporting addresses configured")
		return true, nil
	}

	q := bstore.QueryDB[tlsrptdb.TLSResult](ctx, db)
	if isRcptDom {
		q.FilterNonzero(tlsrptdb.TLSResult{RecipientDomain: policyDomain, DayUTC: dayUTC})
	} else {
		q.FilterNonzero(tlsrptdb.TLSResult{PolicyDomain: policyDomain, DayUTC: dayUTC})
	}
	tlsResults, err := q.List()
	if err != nil {
		return true, fmt.Errorf("get tls results from database: %v", err)
	}

	if len(tlsResults) == 0 {
		// Should not happen. But no point in sending messages with empty reports.
		return true, fmt.Errorf("no tls results found")
	}

	// Stop if we already sent a report for this destination.
	for _, r := range tlsResults {
		if r.PolicyDomain == r.RecipientDomain && (isRcptDom && r.SentToRecipientDomain || !isRcptDom && r.SentToPolicyDomain) {
			return true, nil
		}
	}

	beginUTC := endUTC.Add(-24 * time.Hour)

	report := tlsrpt.Report{
		OrganizationName: fromDom.ASCII,
		DateRange: tlsrpt.TLSRPTDateRange{
			Start: beginUTC,
			End:   endUTC.Add(-time.Second), // Per example, ../rfc/8460:1769
		},
		ContactInfo: "postmaster@" + fromDom.ASCII,
		// todo spec: ../rfc/8460:968 ../rfc/8460:1772 ../rfc/8460:691 subject header assumes a report-id in the form of a msg-id, but example and report-id json field explanation allows free-form report-id's (assuming we're talking about the same report-id here).
		ReportID: endUTC.Add(-12*time.Hour).Format("20060102") + "." + polDom.ASCII + "@" + fromDom.ASCII,
	}

	rcptDomAddresses := map[string][]string{}
	for _, tlsResult := range tlsResults {
		rcptDomAddresses[tlsResult.RecipientDomain] = tlsResult.RecipientDomainReportingAddresses
	}

	// Merge all results into this report.
	// If we are sending to a recipient domain, we include all relevant policy domains,
	// so possibly multiple MX hosts (with DANE policies). That means we may be sending
	// multiple "no-policy-found" results (1 for sts and 0 or more for mx hosts). An
	// explicit no-sts or no-tlsa would make these less ambiguous, but the
	// policy-domain's will make clear which is the MX and which is the recipient
	// domain. Only for recipient domains with an MX target equal to the recipient host
	// could it be confusing.
	// If we are sending to MX targets (that aren't recipient domains), we mention the
	// affected recipient domains as policy-domain while keeping the original policy
	// domain (MX target) in the "mx-host" field. This behaviour isn't in the RFC, but
	// seems useful to give MX operators insight into the recipient domains affected.
	// We also won't include results for a recipient domain if its TLSRPT policy has
	// the same reporting addresses as the MX target TLSRPT policy.
	for i, tlsResult := range tlsResults {
		if !isRcptDom {
			if slices.Equal(rcptDomAddresses[tlsResult.RecipientDomain], recipientStrs) {
				continue
			}
			for j, r := range tlsResult.Results {
				if tlsResult.IsHost {
					tlsResults[i].Results[j].Policy.MXHost = []string{r.Policy.Domain}
				}
				tlsResults[i].Results[j].Policy.Domain = tlsResult.RecipientDomain
			}
		}

		report.Merge(tlsResult.Results...)
	}

	// We may not have any results left, i.e. when this is an MX target and we already
	// sent all results in the report to the recipient domain with identical reporting
	// addresses.
	if len(report.Policies) == 0 {
		return true, nil
	}

	if !mox.Conf.Static.OutgoingTLSReportsForAllSuccess {
		var haveFailure bool
		// Check there is at least one failure. If not, we don't send a report.
		for _, r := range report.Policies {
			if r.Summary.TotalFailureSessionCount > 0 || len(r.FailureDetails) > 0 {
				haveFailure = true
				break
			}
		}
		if !haveFailure {
			return true, nil
		}
	}

	log.Info("sending tls report")

	reportFile, err := store.CreateMessageTemp(log, "tlsreportout")
	if err != nil {
		return false, fmt.Errorf("creating temporary file for outgoing tls report: %v", err)
	}
	defer store.CloseRemoveTempFile(log, reportFile, "generated tls report")

	// ../rfc/8460:905
	gzw := gzip.NewWriter(reportFile)
	enc := json.NewEncoder(gzw)
	enc.SetIndent("", "\t")
	if err == nil {
		err = enc.Encode(report.Convert())
	}
	if err == nil {
		err = gzw.Close()
	}
	if err != nil {
		return false, fmt.Errorf("writing tls report as json with gzip: %v", err)
	}

	msgf, err := store.CreateMessageTemp(log, "tlsreportmsgout")
	if err != nil {
		return false, fmt.Errorf("creating temporary message file with outgoing tls report: %v", err)
	}
	defer store.CloseRemoveTempFile(log, msgf, "message with generated tls report")

	// We are sending reports from our host's postmaster address. In a
	// typical setup the host is a subdomain of a configured domain with
	// DKIM keys, so we can DKIM-sign our reports. SPF should pass anyway.
	// todo future: when sending, use an SMTP MAIL FROM that we can relate back to recipient reporting address so we can stop trying to send reports in case of repeated delivery failure DSNs.
	from := smtp.Address{Localpart: "postmaster", Domain: fromDom}

	// Subject follows the form from RFC. ../rfc/8460:959
	subject := fmt.Sprintf("Report Domain: %s Submitter: %s Report-ID: <%s>", polDom.ASCII, fromDom, report.ReportID)

	// Human-readable part for convenience. ../rfc/8460:917
	text := fmt.Sprintf(`Attached is a TLS report with a summary of connection successes and failures
during attempts to securely deliver messages to your mail server, including
details about errors encountered. You are receiving this message because your
address is specified in the "rua" field of the TLSRPT record for your
domain/host.

Policy Domain: %s
Submitter: %s
Report-ID: %s
Period: %s - %s UTC
`, polDom, fromDom, report.ReportID, beginUTC.Format(time.DateTime), endUTC.Format(time.DateTime))

	// The attached file follows the naming convention from the RFC. ../rfc/8460:849
	reportFilename := fmt.Sprintf("%s!%s!%d!%d.json.gz", fromDom.ASCII, polDom.ASCII, beginUTC.Unix(), endUTC.Add(-time.Second).Unix())

	// Compose the message.
	msgPrefix, has8bit, smtputf8, messageID, err := composeMessage(ctx, log, msgf, polDom, confDKIM, from, recipients, subject, text, reportFilename, reportFile)
	if err != nil {
		return false, fmt.Errorf("composing message with outgoing tls report: %v", err)
	}
	msgInfo, err := msgf.Stat()
	if err != nil {
		return false, fmt.Errorf("stat message with outgoing tls report: %v", err)
	}
	msgSize := int64(len(msgPrefix)) + msgInfo.Size()

	// Already mark the report as sent. If it won't succeed below, it probably won't
	// succeed on a later retry either. And if we would fail to mark a report as sent
	// after sending it, we may sent duplicates or even get in some kind of sending
	// loop.
	err = db.Write(ctx, func(tx *bstore.Tx) error {
		if isRcptDom {
			q := bstore.QueryTx[tlsrptdb.TLSResult](tx)
			q.FilterNonzero(tlsrptdb.TLSResult{DayUTC: dayUTC, RecipientDomain: policyDomain})
			_, err := q.UpdateNonzero(tlsrptdb.TLSResult{SentToRecipientDomain: true})
			if err != nil {
				return fmt.Errorf("already marking tls results as sent for recipient domain: %v", err)
			}

			// Also set reporting addresses for the recipient domain results.
			q = bstore.QueryTx[tlsrptdb.TLSResult](tx)
			q.FilterNonzero(tlsrptdb.TLSResult{DayUTC: dayUTC, RecipientDomain: policyDomain})
			_, err = q.UpdateNonzero(tlsrptdb.TLSResult{RecipientDomainReportingAddresses: recipientStrs})
			if err != nil {
				return fmt.Errorf("storing recipient domain reporting addresses: %v", err)
			}
		} else {
			q := bstore.QueryTx[tlsrptdb.TLSResult](tx)
			q.FilterNonzero(tlsrptdb.TLSResult{DayUTC: dayUTC, PolicyDomain: policyDomain})
			_, err := q.UpdateNonzero(tlsrptdb.TLSResult{SentToPolicyDomain: true})
			if err != nil {
				return fmt.Errorf("already marking tls results as sent for policy domain: %v", err)
			}
		}
		return nil
	})
	if err != nil {
		return false, fmt.Errorf("marking tls results as sent: %v", err)
	}

	var queued bool
	for _, rcpt := range recipients {
		// If recipient is on suppression list, we won't queue the reporting message.
		q := bstore.QueryDB[tlsrptdb.TLSRPTSuppressAddress](ctx, db)
		q.FilterNonzero(tlsrptdb.TLSRPTSuppressAddress{ReportingAddress: rcpt.Address.Path().String()})
		q.FilterGreater("Until", time.Now())
		exists, err := q.Exists()
		if err != nil {
			return false, fmt.Errorf("querying suppress list: %v", err)
		}
		if exists {
			log.Info("suppressing outgoing tls report", slog.Any("reportingaddress", rcpt.Address))
			continue
		}

		qm := queue.MakeMsg(mox.Conf.Static.Postmaster.Account, from.Path(), rcpt.Address.Path(), has8bit, smtputf8, msgSize, messageID, []byte(msgPrefix), nil)
		// Don't try as long as regular deliveries, and stop before we would send the
		// delayed DSN. Though we also won't send that due to IsTLSReport.
		// ../rfc/8460:1077
		qm.MaxAttempts = 5
		qm.IsTLSReport = true
		// TLS failures should be ignored. ../rfc/8460:317 ../rfc/8460:1050
		no := false
		qm.RequireTLS = &no

		err = queueAdd(ctx, log, &qm, msgf)
		if err != nil {
			tempError = !queued
			log.Errorx("queueing message with tls report", err)
			metricReportError.Inc()
		} else {
			queued = true
			tempError = false
			log.Debug("tls report queued", slog.Any("recipient", rcpt))
			metricReport.Inc()
		}
	}

	// Regardless of whether we queued a report, we are not going to keep the
	// evaluations around. Though this can be overridden if tempError is set.
	// ../rfc/7489:1785

	return true, nil
}

func composeMessage(ctx context.Context, log mlog.Log, mf *os.File, policyDomain dns.Domain, confDKIM config.DKIM, fromAddr smtp.Address, recipients []message.NameAddress, subject, text, filename string, reportFile *os.File) (msgPrefix string, has8bit, smtputf8 bool, messageID string, rerr error) {
	xc := message.NewComposer(mf, 100*1024*1024)
	defer func() {
		x := recover()
		if x == nil {
			return
		}
		if err, ok := x.(error); ok && errors.Is(err, message.ErrCompose) {
			rerr = err
			return
		}
		panic(x)
	}()

	// We only use smtputf8 if we have to, with a utf-8 localpart. For IDNA, we use ASCII domains.
	for _, a := range recipients {
		if a.Address.Localpart.IsInternational() {
			xc.SMTPUTF8 = true
			break
		}
	}

	xc.HeaderAddrs("From", []message.NameAddress{{Address: fromAddr}})
	xc.HeaderAddrs("To", recipients)
	xc.Subject(subject)
	// ../rfc/8460:926
	xc.Header("TLS-Report-Domain", policyDomain.ASCII)
	xc.Header("TLS-Report-Submitter", fromAddr.Domain.ASCII)
	// TLS failures should be ignored. ../rfc/8460:317 ../rfc/8460:1050
	xc.Header("TLS-Required", "No")
	messageID = fmt.Sprintf("<%s>", mox.MessageIDGen(xc.SMTPUTF8))
	xc.Header("Message-Id", messageID)
	xc.Header("Date", time.Now().Format(message.RFC5322Z))
	xc.Header("User-Agent", "mox/"+moxvar.Version)
	xc.Header("MIME-Version", "1.0")

	// Multipart message, with a text/plain and the report attached.
	mp := multipart.NewWriter(xc)
	// ../rfc/8460:916
	xc.Header("Content-Type", fmt.Sprintf(`multipart/report; report-type="tlsrpt"; boundary="%s"`, mp.Boundary()))
	xc.Line()

	// Textual part, just mentioning this is a TLS report.
	textBody, ct, cte := xc.TextPart(text)
	textHdr := textproto.MIMEHeader{}
	textHdr.Set("Content-Type", ct)
	textHdr.Set("Content-Transfer-Encoding", cte)
	textp, err := mp.CreatePart(textHdr)
	xc.Checkf(err, "adding text part to message")
	_, err = textp.Write(textBody)
	xc.Checkf(err, "writing text part")

	// TLS report as attachment.
	ahdr := textproto.MIMEHeader{}
	ct = mime.FormatMediaType("application/tlsrpt+gzip", map[string]string{"name": filename})
	ahdr.Set("Content-Type", ct)
	cd := mime.FormatMediaType("attachment", map[string]string{"filename": filename})
	ahdr.Set("Content-Disposition", cd)
	ahdr.Set("Content-Transfer-Encoding", "base64")
	ap, err := mp.CreatePart(ahdr)
	xc.Checkf(err, "adding tls report to message")
	wc := moxio.Base64Writer(ap)
	_, err = io.Copy(wc, &moxio.AtReader{R: reportFile})
	xc.Checkf(err, "adding attachment")
	err = wc.Close()
	xc.Checkf(err, "flushing attachment")

	err = mp.Close()
	xc.Checkf(err, "closing multipart")

	xc.Flush()

	selectors := mox.DKIMSelectors(confDKIM)
	for i, sel := range selectors {
		// Also sign the TLS-Report headers. ../rfc/8460:940
		sel.Headers = append(append([]string{}, sel.Headers...), "TLS-Report-Domain", "TLS-Report-Submitter")
		selectors[i] = sel
	}

	dkimHeader, err := dkim.Sign(ctx, log.Logger, fromAddr.Localpart, fromAddr.Domain, selectors, smtputf8, mf)
	xc.Checkf(err, "dkim-signing report message")

	return dkimHeader, xc.Has8bit, xc.SMTPUTF8, messageID, nil
}