Need help setting up a proxy server?

Background:

We are starting a migration from an internally hosted Exchange 2003 environment to Office 365. In order to do this the vender setup a hybrid server that will use Exchange 2010 since 2003 can migrate to 2010 by cannot migrate to office 365.

We have two firewalls with a DMZ. The hybrid server was initially going to be in the DMZ, however the Swiss cheese set of ports that needed to be opened so it could talk to the domain was too much of a risk so it was moved inside. I do not administer the firewalls.

It must have an external IP address so they can connect to the server. A NAT has been established in the external firewall for this. The hybrid server was originally configured to use the NAT address.

Since we moved it from the DMZ I need to setup a reverse proxy so that any request that is received on the external interface and passed by the firewall will be received by the proxy will be forwarded to the hybrid server IP address. The ports are restricted by the firewall to only include web and SMTP protocols.

I did find several articles for using Application Request Routing on Windows 2012, however that looks like it is only for http and https.

I can use any version of Windows server 2003 and up. The current install is Windows server 2012 but since it is a VM and takes about 10 minutes to build a new one it does not matter to me if another version of Windows server would work better.

Can anyone point me in the right direction to set this up?