Trilent Mail Proxy   
http://www.trilent.net

This firewall-friendly mail (SMTP/POP3) proxy enables small network users to access and distribute email via a single Internet connection, eliminating the need for dedicated mail servers. This is an application-level proxy. It enhances network security by enforcing mail protocol rules and offering many security promoting features, such as access control and event logging. Once configured, the proxy works unobtrusively in the background and requires minimal attention. It offers "install-and-forget" operation. The proxy is designed with reliability in mind. Its modular, multi-threaded design and implementation as a Windows service contributes to its superior stability (it carries the "Designed for Windows" logo.) The proxy supports up to 30 concurrent mail connections, offers extensive compliance with Internet standards and includes a simple mail monitor, capable of monitoring mail connections. Note: To run the program you need a free trial registration key from publisher's Website.