Add comments for missing Message-IDs

src/mailbox/email/mod.rs

@@ -424,6 +424,11 @@ impl Envelope {
                 datetime = Some(value.to_string());
             }
         }
+        /*
+         * https://tools.ietf.org/html/rfc5322#section-3.6.4
+         *
+         * if self.message_id.is_none() { ...
+         */
         if let Some(ref mut x) = in_reply_to {
             self.push_references(x);
         }

src/mailbox/thread.rs

@@ -123,6 +123,9 @@ fn build_collection(
     for (i, x) in collection.iter_mut().enumerate() {
         let x_index; /* x's index in threads */
         let m_id = x.get_message_id_raw().to_string();
+        /* TODO: Check for missing Message-ID.
+         * Solutions: generate a hidden one
+         */
         if id_table.contains_key(&m_id) {
             let t = id_table[&m_id];
             /* the already existing Container should be empty, since we're