fix: return announcement refusals as prepare-phase send errors, test the List-Unsubscribe pair on the wire

mail/announcement.go

@@ -50,20 +50,31 @@ func RenderAnnouncement(a Announcement, publicURL string) (subject, html, text s
 // SendAnnouncement sends one announcement to one address, with the
 // List-Unsubscribe pair Gmail and Yahoo require of bulk senders.
 func (s Sender) SendAnnouncement(to string, a Announcement) error {
+	m, err := s.announcementFor(to, a)
+	if err != nil {
+		return err
+	}
+	return s.send(m)
+}
+
+// announcementFor builds the one-recipient message SendAnnouncement puts on
+// the wire. Every refusal is a *SendError at PhasePrepare: it is about this
+// one message, never the mail server, so a caller moves on to the next row.
+func (s Sender) announcementFor(to string, a Announcement) (message, error) {
 	if strings.Contains(to, ",") {
-		return fmt.Errorf("mail: an announcement goes to exactly one address")
+		return message{}, &SendError{Phase: PhasePrepare, Err: fmt.Errorf("mail: an announcement goes to exactly one address")}
 	}
 	if a.UnsubscribeURL == "" {
-		return fmt.Errorf("mail: announcement without an unsubscribe URL")
+		return message{}, &SendError{Phase: PhasePrepare, Err: fmt.Errorf("mail: announcement without an unsubscribe URL")}
 	}
 	m, err := announcementMessage(a, s.PublicURL)
 	if err != nil {
-		return err
+		return message{}, &SendError{Phase: PhasePrepare, Err: err}
 	}
 	m.To = to
 	m.Headers = map[string]string{
 		"List-Unsubscribe":      "<" + a.UnsubscribeURL + ">",
 		"List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
 	}
-	return s.send(m)
+	return m, nil
 }