Mart,
The easiest way to figure out what is going on is get a packet capture on the SMTP 5 server. If you can reliably reproduce the error I would do as follows.
1. prepare an email to reproduce with
2. Start the capture
3. send the email
4. Wait a minute
5. stop the capture
You should be able to search by ip or even by sender/recipient informaiton in order to find the email and determine what is going on. You can also set the MTA and Filter Engine logs to debug during the test for maximum coverage.