implement "requiretls", rfc 8689

with requiretls, the tls verification mode/rules for email deliveries can be
changed by the sender/submitter. in two ways:

1. "requiretls" smtp extension to always enforce verified tls (with mta-sts or
dnssec+dane), along the entire delivery path until delivery into the final
destination mailbox (so entire transport is verified-tls-protected).

2. "tls-required: no" message header, to ignore any tls and tls verification
errors even if the recipient domain has a policy that requires tls verification
(mta-sts and/or dnssec+dane), allowing delivery of non-sensitive messages in
case of misconfiguration/interoperability issues (at least useful for sending
tls reports).

we enable requiretls by default (only when tls is active), for smtp and
submission. it can be disabled through the config.

for each delivery attempt, we now store (per recipient domain, in the account
of the sender) whether the smtp server supports starttls and requiretls. this
support is shown (after having sent a first message) in the webmail when
sending a message (the previous 3 bars under the address input field are now 5
bars, the first for starttls support, the last for requiretls support). when
all recipient domains for a message are known to implement requiretls,
requiretls is automatically selected for sending (instead of "default" tls
behaviour). users can also select the "fallback to insecure" to add the
"tls-required: no" header.

new metrics are added for insight into requiretls errors and (some, not yet
all) cases where tls-required-no ignored a tls/verification error.

the admin can change the requiretls status for messages in the queue. so with
default delivery attempts, when verified tls is required by failing, an admin
could potentially change the field to "tls-required: no"-behaviour.

messages received (over smtp) with the requiretls option, get a comment added
to their Received header line, just before "id", after "with".

sendmail.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bufio"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -23,18 +24,27 @@ import (
 )
 
 var submitconf struct {
-	LocalHostname      string `sconf-doc:"Hosts don't always have an FQDN, set it explicitly, for EHLO."`
-	Host               string `sconf-doc:"Host to dial for delivery, e.g. mail.<domain>."`
-	Port               int    `sconf-doc:"Port to dial for delivery, e.g. 465 for submissions, 587 for submission, or perhaps 25 for smtp."`
-	TLS                bool   `sconf-doc:"Connect with TLS. Usually for connections to port 465."`
-	STARTTLS           bool   `sconf-doc:"After starting in plain text, use STARTTLS to enable TLS. For port 587 and 25."`
-	Username           string `sconf-doc:"For SMTP authentication."`
-	Password           string `sconf-doc:"For password-based SMTP authentication, e.g. SCRAM-SHA-256, SCRAM-SHA-1, CRAM-MD5, PLAIN."`
-	AuthMethod         string `sconf-doc:"If set, only attempt this authentication mechanism. E.g. SCRAM-SHA-256. If not set, any mutually supported algorithm can be used, in order of most to least secure."`
-	From               string `sconf-doc:"Address for MAIL FROM in SMTP and From-header in message."`
-	DefaultDestination string `sconf:"optional" sconf-doc:"Used when specified address does not contain an @ and may be a local user (eg root)."`
+	LocalHostname      string           `sconf-doc:"Hosts don't always have an FQDN, set it explicitly, for EHLO."`
+	Host               string           `sconf-doc:"Host to dial for delivery, e.g. mail.<domain>."`
+	Port               int              `sconf-doc:"Port to dial for delivery, e.g. 465 for submissions, 587 for submission, or perhaps 25 for smtp."`
+	TLS                bool             `sconf-doc:"Connect with TLS. Usually for connections to port 465."`
+	STARTTLS           bool             `sconf-doc:"After starting in plain text, use STARTTLS to enable TLS. For port 587 and 25."`
+	Username           string           `sconf-doc:"For SMTP authentication."`
+	Password           string           `sconf-doc:"For password-based SMTP authentication, e.g. SCRAM-SHA-256, SCRAM-SHA-1, CRAM-MD5, PLAIN."`
+	AuthMethod         string           `sconf-doc:"If set, only attempt this authentication mechanism. E.g. SCRAM-SHA-256. If not set, any mutually supported algorithm can be used, in order of most to least secure."`
+	From               string           `sconf-doc:"Address for MAIL FROM in SMTP and From-header in message."`
+	DefaultDestination string           `sconf:"optional" sconf-doc:"Used when specified address does not contain an @ and may be a local user (eg root)."`
+	RequireTLS         RequireTLSOption `sconf:"optional" sconf-doc:"If yes, submission server must implement SMTP REQUIRETLS extension, and connection to submission server must use verified TLS. If no, a TLS-Required header with value no is added to the message, allowing fallback to unverified TLS or plain text delivery despite recpient domain policies. By default, the submission server will follow the policies of the recipient domain (MTA-STS and/or DANE), and apply unverified opportunistic TLS with STARTTLS."`
 }
 
+type RequireTLSOption string
+
+const (
+	RequireTLSDefault RequireTLSOption = ""
+	RequireTLSYes     RequireTLSOption = "yes"
+	RequireTLSNo      RequireTLSOption = "no"
+)
+
 func cmdConfigDescribeSendmail(c *cmd) {
 	c.params = ">/etc/moxsubmit.conf"
 	c.help = `Describe configuration for mox when invoked as sendmail.`
@@ -157,6 +167,9 @@ binary should be setgid that group:
 				if !haveTo {
 					line = fmt.Sprintf("To: <%s>\r\n", recipient) + line
 				}
+				if submitconf.RequireTLS == RequireTLSNo {
+					line = "TLS-Required: No\r\n" + line
+				}
 				header = false
 			} else if header {
 				t := strings.SplitN(line, ":", 2)
@@ -199,6 +212,9 @@ binary should be setgid that group:
 			break
 		}
 	}
+	if header && submitconf.RequireTLS == RequireTLSNo {
+		sb.WriteString("TLS-Required: No\r\n")
+	}
 	msg := sb.String()
 
 	if recipient == "" {
@@ -262,6 +278,8 @@ binary should be setgid that group:
 		tlsMode = smtpclient.TLSStrictImmediate
 	} else if submitconf.STARTTLS {
 		tlsMode = smtpclient.TLSStrictStartTLS
+	} else if submitconf.RequireTLS == RequireTLSYes {
+		xsavecheckf(errors.New("cannot submit with requiretls enabled without tls to submission server"), "checking tls configuration")
 	}
 
 	ourHostname, err := dns.ParseDomain(submitconf.LocalHostname)
@@ -277,7 +295,7 @@ binary should be setgid that group:
 	client, err := smtpclient.New(ctx, mlog.New("sendmail"), conn, tlsMode, ourHostname, remoteHostname, auth, nil, nil, nil)
 	xsavecheckf(err, "open smtp session")
 
-	err = client.Deliver(ctx, submitconf.From, recipient, int64(len(msg)), strings.NewReader(msg), true, false)
+	err = client.Deliver(ctx, submitconf.From, recipient, int64(len(msg)), strings.NewReader(msg), true, false, submitconf.RequireTLS == RequireTLSYes)
 	xsavecheckf(err, "submit message")
 
 	if err := client.Close(); err != nil {