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()

internal/mail/send_test.go

@@ -100,6 +100,36 @@ func TestRecipientsCombinesAllFields(t *testing.T) {
 	}
 }
 
+func TestEnvelopeFromStripsDisplayName(t *testing.T) {
+	cases := map[string]string{
+		"Steve Cliff <me@stevecliff.com>": "me@stevecliff.com",
+		"me@stevecliff.com":               "me@stevecliff.com",
+		"<me@stevecliff.com>":             "me@stevecliff.com",
+		"not a valid address":             "not a valid address", // unparseable ⇒ passthrough
+	}
+	for in, want := range cases {
+		if got := envelopeFrom(in); got != want {
+			t.Fatalf("envelopeFrom(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestBuildMIMEKeepsDisplayNameInHeader(t *testing.T) {
+	raw, err := BuildMIME(OutgoingMessage{
+		From:     "Steve Cliff <me@stevecliff.com>",
+		To:       []string{"you@example.com"},
+		Subject:  "hi",
+		BodyText: "body",
+		Date:     time.Date(2026, 6, 23, 12, 0, 0, 0, time.UTC),
+	})
+	if err != nil {
+		t.Fatalf("BuildMIME: %v", err)
+	}
+	if !strings.Contains(string(raw), "Steve Cliff") {
+		t.Fatalf("From header lost display name:\n%s", raw)
+	}
+}
+
 func TestReadHeaderParsesReferences(t *testing.T) {
 	raw := "From: a@x.com\r\n" +
 		"To: b@x.com\r\n" +