Ok. Found the solution and the cause but not why it happends. At least it's a start.
zimbramon sends mail notfications stored on the zimbra.log . Somehow a bad upgrade made logrotate to stop working on this log. zimbra.log became 2Gb in size and it actually stored a lot of past events.
The solution for this was to remove old zimbra.log, change it's permission to zimbra.zimbra and then reinitialize logrotate.
Everything is working fine now.
Why this happens? I have no clue. But will keep looking as soon as i have some extra free time.
$ rm -rf /var/log/zimbra.log
$ touch /var/log/zimbra.log
$ chown zimbra.zimbra /var/log/zimbra.log