For VPOP3 to log outbound messages it needs to be able to work out that the message is FROM: a user whose mail is handled by the local VPOP3. To do this it has to examine the FROM: address in the message header.

If the message is FROM: fred@domain.com, VPOP3 will look to see if fred@domain.com is a local user or not.

This means that first of all, VPOP3 will check that 'domain.com' is listed in the Local Domains box on the Local Mail tab, and secondly, that 'fred' is listed in the VPOP3 user list.