No changes to Exchange were made (Exchange server was a little low on disk space, but still had sufficient space; after increasing disk space and doing regular updates it still doesn’t work, so changes to Exchange didn’t have an effect.
The system stopped fetching mails after doing the upgrade of Ubuntu and Zammad and a reboot.
Could it be that the Ubuntu upgrade changed TLS settings and is less forgiving than the previous version? The ticket below discusses TLS, but the error message is different from ours.
After reading the docs again (I wanted to switch to POP3 because IMAP didn’t work), I read about port 993 for IMAP. We had 143 configured - which was working fine for 1.5 years, and after setting it to 993 Zammad now works again.
Was that announced somewhere in release notes? Probably I missed it.
To help solve this mystery: No, there was no change in “ports we require” as this is absolutely depending on your. However, what we did is we dropped no longer supported / suggested encryptions. Especially everything with SSL and I think TLSv1.0 as well.
This usually isn’t an issue, but may require you to upgrade the cyphers on your exchange server (or update it). Not entirely sure on the exchange end.
The reason why port 993 does work instead of 143 proberbly is the encryption being used or possible.