In some situations you will have a situation where most of your users are handled by VPOP3, but several users will collect their mail directly from a separate POP3 account on your ISP, even though they may have an email address on the same domain as other users.
For instance, if your domain is domain.com, you may have VPOP3 users fred, kate and jim who are all defined as users in VPOP3, but you may also have another user sarah who gets her mail directly from a separate POP3 account on the ISP.
If you don't tell VPOP3 about the sarah user, then, if a local user tries to send mail to firstname.lastname@example.org, VPOP3 will immediately bounce back the message saying that sarah is an unrecognised user name. Also, if an external user sends a message to email@example.com and CC's it to firstname.lastname@example.org, then the VPOP3 administrator will get an error message about an incoming message for email@example.com since that address is not recognised.
To get around this problem, VPOP3 has the concept of Remote Users. If you create a Mapping in VPOP3 of sarah → *REMOTE, then VPOP3 knows that firstname.lastname@example.org exists, but is managed somewhere else. This solves both problems mentioned above. VPOP3 sends locally sent messages to *REMOTE mappings out to the ISP, and ignores incoming messages to *REMOTE mappings.
You create the mappings on the Mappings page in the VPOP3 settings.
If you are using your ISP's domain in your email addresses, so that, for instance, your email address is 'email@example.com', then you will not be able to create mappings of all the other ISP users to *REMOTE as specified above.
In this case, you need to tell VPOP3 that all unrecognised users are remote. To do this, go to Settings → Local Mail → General and check the box
Treat locally sent mail for unrecognised local recipients as outgoing mail