yes, you would have to setup an internet DNS name like mail.companyname.com and external IP address that points to your firewall, or router’s NAT address that is translated to your internal address in the Network Address Translation section of the router configuration.
Then setup a rule on your router that if an https connection comes in from that address to port forward it to your exchange server. You probablt already have similar rules for port 25 (SMTP) if you are delivering external mail directly to your exchange server without a intermediary spam filter or antivirus server.
Essentially, your exchange server is doing BOTH roles of the front end server and the backend server. This is perfectly acceptable in smaller installations.