If you have a catchall filter rule in place to allow general access from your Internal network to the outside world, then mail access should not be a problem. The following filter rule does the trick:
Internal_Network Any Any Allow
If you are using more restrictive filter rules, then you are preventing the machines on your Internal network to access services in the outside world that you have not explicitly allowed for in your packet filtering rules.
If you have a catchall filter rule in place to allow general access from your Internal network to the outside world, then mail access should not be a problem. The following filter rule does the trick:
Internal_Network Any Any Allow
If you are using more restrictive filter rules, then you are preventing the machines on your Internal network to access services in the outside world that you have not explicitly allowed for in your packet filtering rules.