I found out, that my mail did not get delivered to the mailbox for some hours. After further investigation I figured out, that kopano presence was on (usually off). Also I could not send emails anymore, it stated “could not save mail”. I switched kopano presence off and restarted kopano server. Then everything was working fine again. Though, all mails delivered to kopano in that time frame did not reach the mailboxes of the users (postfix delivered the mail to kopano successfully).
Where could I find emails delivered to kopano, but not appearing in the mail boxes?