"A secure reverse proxy to publish Exchange is a must-have for many companies. Historically, the recommended go-to has been Microsoft's Forefront TMG Server, a combination firewall, VPN, Web proxy and reverse-proxy product. At the end of 2012, however, Microsoft announced it was discontinuing the product. Whether you've already deployed TMG, or are looking for a new reverse proxy, a simple question stands out: "What do I do now?"
Now, before examining reverse-proxy alternatives, it's important to distinguish between a load balancer and a reverse proxy. The primary job of a load balancer is to spread incoming connections among multiple application servers. The primary job of a reverse proxy is to terminate incoming connections from untrusted networks and to create a new connection into the application server.
This line is blurring today, and it's important to understand that both feature sets are often found in the same appliances and software packages. For example, Forefront Threat Management Gateway (TMG) is technically a reverse proxy, but the Web farm functionality provides HTTPS load balancing, as well.
Now, onto those alternatives:..."
For the rest of the story, check out http://searchexchange.techtarget.com/tip/Replacing-Forefront-TMG-Alternate-reverse-proxy-options-for-Exchange