Pigeonhole duplicate Message-ID causes forwarded mail to be classified as spam

[neagix]

I have been trying to find out for a long time why emails I forward via SRS are being classified as spam on recipient end (Gmail etc).

The source of the issue is that Pigeonhole adds a duplicate Message-ID header; the bug is quite old (2022) and it has been fixed in Pigeonhole v0.5.21 (2023-08-15).

In fact this is a regression for Docker mailserver: v13.3 used Dovecot 2.3.21 (shipping Pigeonhole v0.5.21, not affected by the bug) via community repository, so when moving to Debian 12 in v14.0.0 the fallback on Bookworm’s Dovecot 2.3.19 (shipping Pigeonhole v0.5.19) caused this bug to come back to life.

In my case I patched DMS' Pigeonhole this way:

--- a/src/lib-sieve/cmd-redirect.c
+++ b/src/lib-sieve/cmd-redirect.c
@@ -529,10 +529,16 @@ static int act_redirect_execute(const struct sieve_action_exec_env *aenv,
 	/* Create Message-ID for the message if it has none */
 	trans->msg_id = msgdata->id;
 	if (trans->msg_id == NULL) {
 		pool_t pool = sieve_result_pool(aenv->result);
-		trans->msg_id = trans->new_msg_id =
-			p_strdup(pool, sieve_message_get_new_id(svinst));
+		const char *msg_id;
+
+		if (mail_get_first_header(msgdata->mail, "Message-ID", &msg_id) > 0)
+			trans->msg_id = p_strdup(pool, msg_id);
+		else {
+			msg_id = sieve_message_get_new_id(svinst);
+			trans->msg_id = trans->new_msg_id = p_strdup(pool, msg_id);
+		}
 	}

 	/* Create ID for duplicate database lookup */

And SRS-forwarded emails immediately stopped being classified as spam. This patch will not be necessary anymore once we have a more recent DMS with a more recent Pigeonhole; let me know if it helps anyone else!