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The Internet Mail Service can intercept and selectively reroute inbound messages from SMTP hosts, Post Office Protocol version 3 (POP3) clients, and Internet Message Access Protocol, Version 4rev1 (IMAP4rev1) clients. These messages are selectively rerouted to other SMTP hosts before they are processed by the Internet Mail Service. You can use the Routing property page to enable the Internet Mail Service to act as a smart host that can route messages between the Internet and other SMTP hosts without the need to define custom recipients. If mail from POP3 clients is to be routed to recipients outside the Microsoft Exchange Server organization, you must configure the Internet Mail Service to reroute mail.
If your organization uses multiple SMTP hosts, you can use the Routing property page to designate the Internet Mail Service as the single point of contact to the Internet. Other SMTP hosts can be configured to forward all messages to the Internet Mail Service, where the Routing property page has been configured to route messages appropriately.
The Routing property page contains a list of domain names and associated SMTP hosts. For each recipient of every inbound message the Internet Mail Service receives, the Routing property page compares the domain name on the address to the list of domain names in the Routing property page. If a match is found, the message for that particular recipient is rerouted to the associated host.
The match is compared to the rightmost part of the domain name in the recipient address. Only full subdomains are compared. For example, if the domain name in the table is sea.com, it matches an address of x.sea.com as well as sea.com, but not chelsea.com. The list is searched from the most specific match to the least specific match. For example, firstname.lastname@example.org would match x.sea.com, sea.com, or .com. However, if x.sea.com is in the list, it is used because it is the most specific match. This occurs regardless of its placement in the list.
In cases where the associated SMTP host is set for <inbound>, domain names in the list represent names that should be handled by the Internet Mail Service. If the recipient address matches one of these domain names, the Internet Mail Service processes the recipient normally and attempts to deliver the message to the Microsoft Exchange Server. If no match is found, the Internet Mail Service reroutes the message outbound to the domain name in the recipient's address. For this reason, it is important to ensure that any domain names that must be routed to the Microsoft Exchange Server are listed in the Routing property page. In particular, any domain names used in SMTP addresses for Microsoft Exchange users must be defined in the Routing property page. Otherwise, reports and replies will not be deliverable and could cause routing loops.
For more information on using the Routing property page, see Microsoft Exchange Server Operations.