If you have a router that supports port forwarding, you can forward the appropriate traffic through to your mailserver.
A word of caution: If you set up your mailserver to forward indiscriminately, it will be found and used to forward loads of spam. It is only a matter of time before it gets discovered and exploited.
This tends to get back to your ISP, with serious consequences. It is much better to use a local mailserver for outgoing mail, wherever you may be. If you are at a friend's place, for instance, and he has comcast, use the comcast server for outgoing. It is open to forwarding from withing its range of IP's. You cans still retrieve your mail safely, just enable forwarding from a very small range of IP's.