You can define a program which you want to use to tell VPOP3 which user(s) to send incoming message to. You set this on the VPOP3 Extensions page. This program will be run whenever a message is downloaded by VPOP3 using the POP3 mail protocol.
Virus check incoming message attachments
In VPOP3 V2 and later, we recommend that Message Processor plugins are created instead of External Routers, as Message Processor plugins work with SMTP messages as well as POP3 messages. They also have more flexibility and can perform common tasks more simply than External Routers.