Add a default Message-ID when sending emails (#5495)

crates/email/src/mailer.rs

@@ -53,6 +53,9 @@ impl Mailer {
         Message::builder()
             .from(self.from.clone())
             .reply_to(self.reply_to.clone())
+            // By passing `None`, lettre generates a random message ID
+            // with a random UUID and the hostname for us
+            .message_id(None)
     }
 
     fn prepare_verification_email(