Upgrade dovecot to 2.4.x from 2.3

The new year holidays brings joice and excitement with a little spared time and when ubuntu suggests to upgrade to 25.10, I started doing that without hesitation.

Everything was run smoothly upto the end and there was a warning that the dovecot service was not restarted successfully.

After trying few time with no lucks, I started to check its details and it shown the error in line x of dovecot.conf file. WTF? I after correct one, it poped up another. I did google for the errors then discovered that other had the same issue, particularly who are running ispconfig

My questions then was to rollback Dovecot to 2.3 or use it and fix the issue? The former seems much more comfortable because it had been running for years but the latter should be better because it is at least avoiding zero-day exploition and best practices.

And it was during holidays time, so I deciced to take the latter route. After few hours research, I had it run but no emails can be received and so outgoing ones.

With the help of verbose debug, I finally get it works properly.

[private/dovecot-lmtp] said: 550 5.1.1 User doesn't exist: xxx@yyy.net (in reply to RCPT TO command))