Updated to Ubuntu 14.04, mail stopped working

==== Required information ====
- iRedMail version: 0.9.2
- Store mail accounts in MySQL
- Web server: Apache
- Linux/BSD distribution name and version: Ubuntu 14.04
- Related log if you're reporting an issue: ?
====

Today I updated Ubuntu from 12.04 to 14.04 to get a more recent PHP version. E-mail stopped working.

When I try to send mail from Thunderbird it says "The mail server responded: 4.3.5 Server configuration problem".

When I click "Get Messages" it seems to validate the password correctly, but no new e-mails arrive (I can see some new e-mails in the logs).

I'm no expert, so I don't know where to look. I haven't seen any obvious errors in the logs, and I see amavisd, dovecot, lmtp, clamd and smtpd running.

Where can I start debugging this?

Update:

iredapd.log shows e-mails arriving like this:

DATE TIME INFO [IP] RCPT, FROM -> TO, DUNNO

dovecot.log shows me checking the e-mail:

DATE TIME pop3-login: Info: Login: user=<mymail@domain.com>, method=PLAIN, rip=XX.XX.XX.XX, lip=XX.XX.XX.XX, mpid=57XX, TLS, session=<.....>
DATE TIME pop3(mymail@domain.com): Info: Disconnected: Logged out top=0/0, retr=0/0, del=0/1, size=344552

dovecot-sieve.log shows:

... saved mail to INBOX