There are many commercial firewall products that provide proxy services. Some of these support Simple Mail Transfer Protocol (SMTP) e-mail. There are also free implementations of SMTP proxies, such as smap, which forward messages between internal and external systems. These products are not likely to have security weaknesses because they are typically designed solely for security purposes. Furthermore, they are usually a simple implementation with restricted functionality. However, a disadvantage of firewall software is that different SMTP servers must be managed. If your internal SMTP server is upgraded with new features, the firewall software must also be upgraded.
Because Microsoft Exchange Server provides secure Internet access, additional firewall software is unnecessary. However, you can use Internet Mail Service with firewall software. To do so, configure your Internet Mail Service to forward all mail to the bastion host running the firewall software.