Nextcloud Mail issues

[WiseMetalhead]

Strange, but Nextcloud Mail suddenly stopped working:

[index] Ошибка: Error connecting to mail server.
	GET /apps/mail/api/accounts/1/quota
	на 31.173.80.134 от vladislav в 15 окт. 2025 г., 18:56:28

[mail] Ошибка: Cron mail sync failed for account 1
	на ? от -- в 15 окт. 2025 г., 19:40:48

If I connect from external apps like Thunderbird Mobile, everything works as expected. Snappy Mail app also works fine.
But when I try to log in to the mail server from Nextcloud Mail (both the latest version and 5.4.0), this error appears in the Mail log:

Oct 15 19:58:45 [NOTICE] [FF7E0883-2134-4B12-8030-062F0017AB0A] [core] connect ip=158.94.209.47 port=58086 local_ip=172.18.30.4 local_port=2587
Oct 15 19:58:45 [INFO] [FF7E0883-2134-4B12-8030-062F0017AB0A] [dns-list]
Oct 15 19:58:46 [INFO] [FF7E0883-2134-4B12-8030-062F0017AB0A] [helo.checks] helo_host: User, pass:bare_ip, rdns_match(org_dom), host_mismatch, literal_mismatch, fail:valid_hostname(no_dot), skip:dynamic(no dots)
Oct 15 19:58:46 [INFO] [FF7E0883-2134-4B12-8030-062F0017AB0A] [spf] identity=helo ip=158.94.209.47 domain="User" mfrom=<postmaster@User> result=None
Oct 15 19:58:46 [INFO] [FF7E0883-2134-4B12-8030-062F0017AB0A] [spf] scope: helo, result: None, domain: User
Oct 15 19:58:46 [INFO] [FF7E0883-2134-4B12-8030-062F0017AB0A] [limit] concurrent_count: 1, concurrent: 1/200, rate_conn: 1:60/1m, unrec_cmds: AUTH,LOGIN
Oct 15 19:58:46 [NOTICE] [FF7E0883-2134-4B12-8030-062F0017AB0A] [core] disconnect ip=158.94.209.47 rdns=NXDOMAIN helo=User relay=N early=N esmtp=Y tls=N pipe=N errors=1 txns=0 rcpts=0/0/0 msgs=0/0/0 bytes=0 lr="500 Unrecognized command" time=1.391

I’m not sure if this is somehow related to the Docker image migration, but I didn’t do anything else with Cloudron today.