Yes, BG30 can support it, please see the details setting as below.

This is a simple Network diagram, it will acheive below request

1. To pass SMTP traffic through WAN1.
2. To pass POP3 traffic through WAN2.
3. When some request enters through WAN1, it should go out through WAN1. And when request comes through WAN2, it should go out through WAN2.
4. Can access to mail server from Internet

Step1：do protocol binding function on BiGuard，the details on menu Configuration->Dual WAN->Protocol Binding，all send mail packets through WAN1
 
Step2：do protocol binding function on BiGuard，the details on menu Configuration->Dual WAN->Protocol Binding，all received mail packets through WAN2
 
Step3：do virtual server function on BiGuard，the details on menu Configuration->Virtual Server
 
Step4：do virtual server function on BiGuard，the details on menu Configuration->Virtual Server