Dual NIC cards on Firewall with mailserver

I have a SuSE 8.2 Linux box with 2 interfaces. One is configured with an
ip address for the private internal network (192.168....) The other is
attached to a DSL connection going out into the wider world.

There is a firewall on the machine allowing http, SMTP and IMAPS. I have
Apache 2 and Postfix installed on it.

I can connect to the Mail server internally if I address the account on
the internal IP address and I can connect from outside, over the
internet, if I address it on the external ip interface. However, I have
a laptop which I sometimes connect to internally and sometimes
externally. I want to be able to put in the external ip address and it
still be able to connect to the mail server when I am in the office.
However I cannot be on the internal network (the 192.168... range) and
connect via the internal Interface of the machine to the external
address range. This means I cannot collect mail from "inside" without
continually changing the mail account server settings.

Am I trying to do something I shouldn't! Should I try to remove so many
services from one shared box? Is this an easy problem to overcome? So
many questions. Any pointers appreciated.

Chris