Hello,

First of all, thank you to the whole community of developers and testers for this project, which I appreciate very much.

[In the rest of the post, my domain name will be replaced by domain.tld, the IP of the main MX server with 1.1.1.1 and that of the MX 2 by 2.2.2.2].

I installed in early June 2020 a Mailcow instance on a server with a dedicated IPv4, and a correctly defined PTR record on it.
Then, following this guide, I configured a 2nd server as a secondary MX for my domain, so that it keeps cached emails when the main server encounters problems. However, I didn’t configure relay_recipients on this second server, preferring to let Mailcow take care of this part.

I had not, since these two installations, made any update on these two servers (yes it’s bad).
I updated Mailcow yesterday, and, since then, the secondary server seems not to be able to relay incoming mails anymore: they remain in queue, with the status mail for domain.tld loops back to myself.
From what I understood, this error could be caused by an identical hostname on both servers; however this is not the case, the Mailcow server having as name mail.domain.tld, and the MX 2, mx2.domain.tld.

Translated with www.DeepL.com/Translator (free version)

State of the queue on MX 2 (obtained with postqueue -p):

-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
1A0AD23F08B     3174 Mon Dec  7 11:39:57  some_gmail_user@gmail.com
                                    (mail for domain.tld loops back to myself)
                                         existing_user@domain.tld

Logs on mail.domain.tld when running postqueue -i 1A0AD23F08B on MX 2:

postfix-mailcow_1    | Dec  7 15:10:57 mail postfix/postscreen[1517]: CONNECT from [2.2.2.2]:45902 to [172.22.1.11]:25
php-fpm-mailcow_1    | fd4d:6169:6c63:6f77::d -  07/Dec/2020:15:10:57 +0100 "GET /forwardinghosts.php" 200
nginx-mailcow_1      | fd4d:6169:6c63:6f77::f - - [07/Dec/2020:15:10:57 +0100] "GET /forwardinghosts.php?host=2.2.2.2 HTTP/1.1" 200 20 "-" "curl/7.64.0"
postfix-mailcow_1    | Dec  7 15:10:57 mail whitelist_forwardinghosts: Look up 2.2.2.2 on whitelist, result 200 PERMIT
postfix-mailcow_1    | Dec  7 15:10:57 mail postfix/postscreen[1517]: WHITELISTED [2.2.2.2]:45902
postfix-mailcow_1    | Dec  7 15:10:57 mail postfix/smtpd[1525]: connect from mx2.domain.tld[2.2.2.2]
postfix-mailcow_1    | Dec  7 15:10:58 mail postfix/smtpd[1525]: Anonymous TLS connection established from mx2.domain.tld[2.2.2.2]: TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256
postfix-mailcow_1    | Dec  7 15:10:58 mail postfix/smtpd[1525]: disconnect from mx2.domain.tld[2.2.2.2] ehlo=1 starttls=1 quit=1 commands=3

I attach main.cf of both MX1 (Mailcow server) and MX2 (SImple postfix backup server), if needed.

mx1-main.cf

8kB

mx2-main.cf

2kB

Thanks in advance for your precious help !!

Regards,

Charles