If your ISP is providing you with an SMTP mail feed (incoming SMTP) and you have VPOP3 connecting to the ISP through a router then you will probably need to modify the router configuration to allow this.

Most routers' default configuration will only allow outbound connections (ie connections from the local network out to the Internet). If you need to accept incoming SMTP mail, then you need to tell the router to allow inbound connections on port 25 for the IP address of the mail server.

Your ISP should be able to tell you what IP address they have assigned as the address of your mail server, and you should make sure that this is the address you use.

Because there are so many different sorts of router available and many different ways of configuring them we cannot give the details of how this should be done. You should contact the suppliers of the router who should be able to do this.

Note that most routers will not allow this behaviour (incoming SMTP) if you are using an 'address pooling NAT (Network Address Translation)' setup. They probably will allow you to have incoming SMTP if you are not using NAT or have static NAT translations set up.

If the ISP have specified that your mail server should be on IP address 123.123.123.123, you would need to have something like:

interface BRI0
  ip access-group 110 in
!
access-list 110 permit tcp any host 123.123.123.123 eq smtp

in your configuration.

If the ISP have specified that your mail server should be on IP address 123.123.123.123, and you have assigned this an internal address of 192.168.1.1 you would need to have something like:

interface BRI0
  ip access-group 110 in
!
ip nat inside source static 192.168.1.1 123.123.123.123
access-list 110 permit tcp any host 123.123.123.123 eq smtp

in your configuration.