For VPOP3 to monitor outbound messages it needs to be able to work out that the message is FROM: a user whose mail should be monitored. 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.

• If you have told VPOP3 to Monitor All Outbound Messages, VPOP3 will check the Local Domains setting on the Local Mail tab to see if it contains the domain.com entry
• If you have told VPOP3 to Monitor Selected Outbound Messages, VPOP3 will check the Local Domains as above, and it will also look for a VPOP3 user called Fred, which must have the Monitor Messages option enabled.

Note: if you use some form of SMTP authentication (either POP3 before SMTP authentication or full SMTP authentication), then VPOP3 will use the login name to decide whether it should monitor the message, instead of using the FROM: address. (n.b. some email clients do not support SMTP authentication)