Exchange can share a common SMTP address space with different e-mail systems by using contacts and SMTP connectors to determine the path for the e-mail messages. The domain part of the targetAddress attribute of a contact determines the destination e-mail system. The e-mail addresses that are displayed on the E-mail Addresses tab of the contact determines which e-mail addresses will resolve to the contact. From this information, Exchange can route the e-mail message to the correct targetAddress. The targetAddress corresponds to the e-mail address on the General tab of the contact. Also, the targetAddress is used as the primary SMTP address of the contact. The primary SMTP address is denoted by the "SMTP" acronym that appears in uppercase bold characters before the particular SMTP e-mail address.

When an Exchange system receives an SMTP e-mail message, Exchange queries Active Directory to try to resolve the e-mail address that is listed in the recipient field of the message to the e-mail address of an object in Active Directory. If a match is found, Exchange routes the message to the e-mail address that is specified in the targetAddress attribute of the object. For the purposes of this article, this e-mail address is the e-mail address that is present in a remote e-mail system that has a unique SMTP domain.

This routing feature lets Exchange act as a mail switch for any number of SMTP domains that are hosted by different e-mail systems. The contact has a primary SMTP address that corresponds to the remote messaging system and the contact has a secondary SMTP address that corresponds to the SMTP domain of the Exchange system. You use SMTP connectors that are configured to use address spaces that are specific to the remote e-mail system's domain to route e-mail to the remote e-mail systems.

User accounts in the remote e-mail system must have a primary SMTP e-mail address that corresponds to the shared SMTP address space and a secondary SMTP e-mail address that corresponds to the primary SMTP e-mail address, also known as the targetAddress, of the contact in Active Directory.

In this scenario, the following e-mail exchange occurs:

1.	When a user from the remote e-mail system sends an e-mail message to the Internet, the domain part of the reply address will correspond to the secondary SMTP e-mail address of the contact in Active Directory.
2.	When the user from the Internet replies, the message is routed to the Exchange server.
3.	When Exchange receives the message, Exchange resolves the e-mail address to the secondary SMTP address of the contact in Active Directory.
4.	Exchange routes the message to the primary SMTP e-mail address of the Active Directory contact. This is the targetAddress of the contact. Therefore, the e-mail message is routed to the remote e-mail system.

To use contacts to redirect e-mail to a remote e-mail system, follow these steps.