fix: send the bare address as SMTP envelope sender so mailcow DKIM-signs

mail/sender.go

@@ -16,6 +16,7 @@ import (
 	"mime/multipart"
 	"mime/quotedprintable"
 	"net"
+	netmail "net/mail"
 	"net/smtp"
 	"net/textproto"
 	"os"
@@ -136,11 +137,11 @@ func (s Sender) send(m message) error {
 		}
 	}
 
-	if err := client.Mail(s.From); err != nil {
+	if err := client.Mail(addrSpec(s.From)); err != nil {
 		return fmt.Errorf("smtp: mail from: %w", err)
 	}
 	for _, rcpt := range rcpts {
-		if err := client.Rcpt(rcpt); err != nil {
+		if err := client.Rcpt(addrSpec(rcpt)); err != nil {
 			return fmt.Errorf("smtp: rcpt %s: %w", rcpt, err)
 		}
 	}
@@ -163,6 +164,20 @@ func (s Sender) send(m message) error {
 	return client.Quit()
 }
 
+// addrSpec is the bare address for the SMTP envelope. SMTP_FROM is usually
+// "Vantage <support@example.com>", which belongs in the From header only:
+// sent as MAIL FROM it became "<Vantage <support@example.com>>". Postfix
+// salvaged the address, so delivery and Return-Path looked fine, but rspamd
+// saw no envelope sender and mailcow skipped DKIM signing, so Gmail filed
+// every Vantage email as spam. Anything that does not parse is passed through
+// for the server to judge.
+func addrSpec(v string) string {
+	if a, err := netmail.ParseAddress(v); err == nil {
+		return a.Address
+	}
+	return strings.TrimSpace(v)
+}
+
 func recipients(to string) []string {
 	parts := strings.Split(to, ",")
 	out := make([]string, 0, len(parts))
@@ -184,10 +199,8 @@ func recipients(to string) []string {
 //
 // Both parts are quoted-printable. They were raw UTF-8 with no
 // Content-Transfer-Encoding, which means 7bit, and every template carries
-// non-ASCII (the middot in the masthead at least). rspamd scored that
-// R_BAD_CTE_7BIT, the message went out without a DKIM signature, and Gmail
-// filed it as spam, while mail from the same mailbox via SOGo, sent
-// quoted-printable, was signed and delivered.
+// non-ASCII (the middot in the masthead at least), which rspamd scored as
+// R_BAD_CTE_7BIT.
 func (s Sender) envelope(m message) ([]byte, error) {
 	var parts strings.Builder
 	w := multipart.NewWriter(&parts)