SBS 2003 Exchange missing emails using pop3 connector

We discover missing emails in our SBS 2003 Exchange.

We found the emails at exchange logs (message tracking is enable). Unfortunately  the "Recipient-Address" is only a dash "-" for these emails.

We are getting the emails using pop3 connector from an external IS account. 

We usually expect at "Recipient-Address" email in a form mspop3connector.<myemailaddress>@domain.com

I would appreciate if anyone can help me to identify or debug the problem.

Thanks in advance