There are some exceptions in the log, but far less than the mail that are not migrated. Eg:
This is the only excepion related to one Mailbox (the address is masked...). This mailbox have hundreds of mail missing (total mails 18400, trasferred less than 17500).
I see that there's a TIMEOUT exception at 12:12, but the transfer not stopped at that time, it has finished at about 15:10 (3 PM).
If you want I can give you some acconts in my Kerio servers (even the one in the example) that you can use to try a migration in your test enviroment to see if you can replicate the issue.
P.S.: using ImapSync to transfer the mails form Kerio to SmarterMail ALL the mail are transferred correctly, so I'm not in the hurry to have a FIX, but I want to help you to make SmarterMail better!