I would have to say that you can only do this if you have it backed up...
This wiki page here has a corresponding thread here in the forums that explains it a little bit more, but the wiki should be all you need:
http://www.scalix.com/wiki/index.php?ti ... ckupScript
The problem of course is that you probably haven't done this yet...
If you have a recent backup of the /var/opt/scalix dir then you can restore the whole scalix message store to a test server, make a backup of that user's mailbox, and then import it into your live production server... but that's only if you have a backup of that dir that is anywhere near recent.
Your other option that I can see would be to have that user give you a copy of the PST file from his outlook that he downloaded them into, setup a new outlook profile for that user using Outlook Scalix Connect, and then import the PST into the scalix message store.
If he wasn't using outlook for his pop... find a way to move the messages from whatever client he was using into something that outlook can import, and then import that (once you have outlook connected to scalix using the Outlook Connect client of course).
Good luck!
Mito