As a cost-cutting measure, we want to move mail traffic off the dedicated, private-backbone connections between our regional sites and headquarters. But when we tried the idea out on an SMTP connector for one of our Exchange Server sites, mail stopped flowing. What happened?
SMTP mail flows over the Internet all the time, but within an Exchange organization (and, more particularly, between routing groups), the SMTP in use is actually extended SMTP (ESMTP). Microsoft has added some of its own SMTP verbs, such as X-EXCH50, to ESMTP. That jack-of-all-trades verb is used to exchange link state and connector data, among other things. Non-Exchange servers won't support it, so you can't use non-Exchange servers to pass mail between routing groups. (You can learn more about ESMTP by reading the Windows IT Pro article "Updating SMTP Support," September 2001, InstantDoc ID 21865.) As an alternative to using a non-Exchange server, you can use Microsoft Internet Security and Acceleration (ISA) Server or your favorite hardware firewall to create site-to-site VPNs that will pass your site-to-site SMTP traffic directly between Exchange servers while still using an ordinary Internet connection.