VPOP3 can be configured to send through an SMTP Relay server (sometimes known as a smarthost). This is the usual configuration for small/medium sized businesses and home users. If you have a dial-up, ADSL, Cable, FTTC etc connection, then your Internet Provider will normally give you access to their SMTP relay service.
In some cases, the Internet Provider's SMTP relay service may not be suitable. In that case, you may be able to use the SMTP Direct sending method, or you may be able to subscribe to an SMTP relay service from another provider (this will not require you to change your incoming mail or Internet connection configuration).
The SMTP Relay settings in VPOP3 tell VPOP3 how to send mail through your Internet Provider's SMTP relay service.