GitLab does not check for incoming e-mail

We have GitLab incoming-email setup a while ago to use the reply-by-mail feature and it was working fine.
As it is not much used, we just realized that it doesn’t seem to work anymore.

The check looks ok:

# docker exec -it gitlab gitlab-rake gitlab:incoming_email:check
Checking Incoming Email ...

Incoming Email: ... Checking Reply by email ...

IMAP server credentials are correct? ... Checking gitlab
yes
Init.d configured correctly? ... skipped
MailRoom running? ... skipped

Checking Reply by email ... Finished


Checking Incoming Email ... Finished

But e-mail send to an issue are not received by GitLab. Looking at the log of the imap server, it seems that GitLab does not connect it much. This is all I get from January in the log:

2021-01-12T03:09:54.064523+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=360, TLS, session=<YhBKW6u4+N2sEQAB>
2021-01-12T03:11:49.058077+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=363, TLS, session=<eeEkYqu4gOCsEQAB>
2021-01-12T03:12:51.171229+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=366, TLS, session=<LJTYZau4kOKsEQAB>
2021-01-12T03:12:57.038134+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=368, TLS, session=<PCwyZqu4vuKsEQAB>
2021-01-26T05:31:19.297988+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=615, TLS, session=<irzT9sa5XLysEQAB>
2021-01-26T05:35:49.746165+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=618, TLS, session=<x5DyBse5YsKsEQAB>
2021-01-26T05:36:51.615406+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=621, TLS, session=<romiCse5MsSsEQAB>
2021-01-26T05:36:57.538320+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=623, TLS, session=<Mv38Cse5QsSsEQAB>
2021-01-28T03:48:51.871151+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=745, TLS, session=<6z0YxO25DuasEQAB>
2021-01-28T03:50:31.885956+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=747, TLS, session=<BWwOyu25CuisEQAB>
2021-01-28T03:51:33.843101+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=751, TLS, session=<Hr2/ze25bOmsEQAB>
2021-01-28T03:51:39.790122+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=753, TLS, session=<VY4azu25gOmsEQAB>
2021-01-28T03:52:04.491004+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=756, TLS, session=<8GOTz+25+umsEQAB>
2021-01-28T03:54:02.462443+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=759, TLS, session=<ro6b1u25YuusEQAB>
2021-01-28T07:02:07.574765+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=763, TLS, session=<oaVAd/C57JasEQAB>
2021-01-28T07:15:53.031273+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=766, TLS, session=<1hp0qPC5uKCsEQAB>
2021-01-28T07:15:58.882490+00:00 internal dovecot: imap-login: Login: user=<gitlab>, method=PLAIN, rip=172.17.0.1, lip=172.17.0.6, mpid=768, TLS, session=<13TNqPC5zqCsEQAB>

These logins seems to be from gitlab:incoming_email:check and when GitLab reconfigure is run, e.g., I did an update to GitLab 13.8.1 today.
But I would expect regular logins from GitLab to check for e-mail.

This is from my gitlab.rb:

gitlab_rails['incoming_email_enabled'] = true
gitlab_rails['incoming_email_address'] = "gitlab+%{key}@sub.domain.com"
gitlab_rails['incoming_email_email'] = "gitlab"
gitlab_rails['incoming_email_password'] = "secret"
gitlab_rails['incoming_email_host'] = "sub.domain.com"
gitlab_rails['incoming_email_port'] = 993
gitlab_rails['incoming_email_ssl'] = true
gitlab_rails['incoming_email_start_tls'] = false

Looks like the problem was on the smtp server side and e-mails didn’t reach the imap server.
Still puzzled how the e-mail get to GitLab if it doesn’t seem to check, but maybe the log just don’t tell everything.