When you setup a Kerio server, by default there’s a feature called AutoExpunge. This feature keeps mail clients from showing a message with a strikethrough through it when a message is marked for deletion. Once items are processed the message is moved to deleted and the strikethrough message is removed from the folder it was deleted from. Many users can get confused by this, so Kerio built a feature called AutoExpunge. That AutoExpunge feature instead of striking through messages just tosses them. That causes you to be unable to undo a delete.
To disable AutoExpunge, stop Kerio Mail Server and then look for AutoExpungeOnDelete option in /usr/local/Kerio/mailserver/mailserver.cfg (I like to back that file up before making any changes). Then change the value for that from 1 to 0. Then save your changes to the file and start Kerio Connect back up. Once started, test that you can undo a delete and if so, you’re good to go!
Note: If you change settings like this when the mail server is running then it can revert the settings back as the daemon is running. If that happens to you, double-check that the service is stopped before editing the file.