feat(mail): derive bare envelope sender from display-name From

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/mail/send.go

@@ -46,6 +46,18 @@ func (m OutgoingMessage) Recipients() []string {
 	return out
 }
 
+// envelopeFrom returns the bare address for the SMTP envelope sender, stripping
+// any display name. A display-name From (e.g. "Name <addr>") is a valid header
+// but an invalid envelope sender, so it must be reduced to the bare address.
+// Unparseable input is passed through unchanged (preserves prior behaviour for
+// plain addresses).
+func envelopeFrom(from string) string {
+	if a, err := gomail.ParseAddress(from); err == nil {
+		return a.Address
+	}
+	return from
+}
+
 func addrList(addrs []string) []*gomail.Address {
 	out := make([]*gomail.Address, 0, len(addrs))
 	for _, a := range addrs {
@@ -163,7 +175,7 @@ func SendSMTP(cfg SMTPConfig, m OutgoingMessage) error {
 	if err := c.Auth(auth); err != nil {
 		return fmt.Errorf("smtp auth: %w", err)
 	}
-	if err := c.SendMail(m.From, m.Recipients(), bytes.NewReader(raw)); err != nil {
+	if err := c.SendMail(envelopeFrom(m.From), m.Recipients(), bytes.NewReader(raw)); err != nil {
 		return fmt.Errorf("smtp send: %w", err)
 	}
 	return c.Quit()