In a situation where the dagent crashes on a certain eml the culprit is still in the postfix queue.
As these files are in a proprietary format it is hard to analyze them.
However they can be converted to proper EML with the provided postcat utility
In order to use postcat, first you have to use the postqueue -p command to get the Queue ID required.
$ postqueue -p
In order to convert to EML, first get hold of which file its in the queue which could be provided to Kopano Support for further analysis if required.
$ postcat -bh queueid > file.eml
If you need to send this eml file to Kopano support please add the eml file in a zip file.
Or PIPE the EML direct into the kopano-dagent to try to manually to deliver the mail.
$ postcat -bh queuefile | kopano-dagent username