We found, that if the mail-server is not available for a while (changed password or whatever) and the account gets re-activated, then the new emails received meanwhile don’t get fetched by Zammad. Only new mails when Zammad is online are received.
It is required for Zammad also processing emails from meanwhile when offline. How can we achieve that?
Sounds like someone did mark the mails in question as read and Zammad is configured to keep the messages on the mail server. If that’s the case, mark the mails as unread and you’re good to go.