Mail delivery and SMTP changes in Exchange 2007

As a result of the extensive overhaul that resulted in a redesigned Exchange 2007, many people have questions when it comes to what has changed and why.  An area that is sometimes questioned is the role of SMTP in Exchange 2007.

In Exchange versions prior to Exchange 2007, in order to communicate both with each other and with Internet mail servers, Exchange servers relied solely on the SMTP services provided by IIS.  If you recall, one of the prerequisites for Exchange 2000/2003 was the installation of IIS and IIS' SMTP services.

With Exchange 2007, Microsoft has provided Exchange with its ownSMTP services, thus negating the need for IIS’ SMTP services.  In fact, if you do have IIS’ SMTP services installed, those services will prove to be a conflict.

Further, remember that Exchange 2007 is role-based.  Inter-server communication methods between services varies. The mailbox role, for example, has no SMTP capability and uses only MAPI/RPC to communicate with Hub Transport servers.  Hub Transport (and, if installed, Edge Transport) servers are the SMTP carriers in an Exchange 2007 organization.  All mail—even mail between mailboxes on the same mailbox server—has to hit a Hub Transport server.  Mail sent outside the Exchange organization is sent using SMTP and, of course, incoming mail is received by an Edge or Hub Transport server via SMTP.

The Hub Transport server and mail delivery

Suppose you’re troubleshooting an e-mail delivery problem in Exchange 2007.  You might want to know the route that the message traverses as it makes its way around the organization.  Here's a quick look at how the Hub Transport server and mail delivery work in Exchange 2007.

Understand that every single message that is sent in your Exchange organization passes through a Hub Transport server during its journey.  This holds true even for messages sent between users whose mailboxes resides in the same database on the same mailbox server.  At first blush, this seems incredibly inefficient.  After all, why would you waste network bandwidth and Hub Transport server processing power to handle a message that is just going to

be right back from whence it came?

The answer: Messaging policy compliance.  All of the transport rules that can be applied to messages are applied at the Hub Transport server.  The Mailbox Server role does not hold any of this logic—nor should it.  Therefore, the method utilized by Exchange 2007 is, from a complexity standpoint, much more straightforward.  By the way, the Hub Transport server communicates with a Mailbox Server using MAPI/RPC. If a message is sent between different Hub Transport servers in different Active Directory sites, the transport protocol used is SMTP.  This holds true for communication between an Exchange 2007 Hub Transport server and a legacy Exchange routing group connector.