How to make echange server work inside & outside LAN

I have 1 static IP.

I want echange server to act as a stand alone email server.

Its easy to set it up out of the office LAN because then you use 1 ip address for pop3 & SMTP server settings.

What if i wish to put the exchange server inside my office LAN.

Exhcange server should behave like a pop3 server not an Echange account. Users in outlook add a pop3 server.

This way they can use their emails on the run in or out of the office without VPN.

Maybe i can configure out firewall server to redirect the dns request of static exchange server to point to its internal network address? when a user is inside the office?