Selecting custom ports for mail channel ends in 'host not reachable'

[MrGeneration]

Infos:

• Used Zammad version: stable / 5.0.3 (as of today 4pm available)
• Installation method (source, package, ..): any
• Operating system: any
• Database + version: any
• Elasticsearch version: any
• Browser + version: verified on Opera v83.0.4254.54 (Chromium based)
• Ticket: #10101199

Expected behavior:

Zammad connects to alternative or custom mail ports without issues.

Actual behavior:

Connecting to highport does seem to work on a technical level, as dovecot logs this after roughly 2 minutes after connection try:
Feb 15 16:03:23 cc dovecot: imap-login: Disconnected: Inactivity (no auth attempts in 180 secs): user=<>, rip=XXXX, lip=XXX, TLS handshaking, session=<XXXXX>

Zammad is much quicker in telling the admin that the host is not reachable.
In fact, the host is reachable, connection somehow seems to work (but expect different things (?)).

No useful error messages in production.log available.
Did not verify older versions of Zammad.

This may indirectly belong to #3921
It may also indirectly affect SMTP and POP3 - I only have a IMAP server at hand, SMTP can't be tested as IMAP dialogue already blocks.

Steps to reproduce the behavior:

• have a mail server with a port configuration that does not match default RFC ports
• try to set up your account

Yes I'm sure this is a bug and no feature request or a general question.