The VPOP3 SMTP service provides SMTP services allowing local users to send mail, as well as incoming messages via SMTP (if your firewall is configured appropriately and your domain MX records are set accordingly).
In VPOP3 Enterprise, you can create multiple SMTP services with different settings. This would allow, for instance, you to create an SMTP service on port 25 for incoming mail, and an SMTP Submission service on port 587 for authenticated users to send outgoing mail. To create multiple SMTP services, go to the General services page and press the Add SMTP Server button.