In Exchange 2007 a mail-enabled contact represents a recipient outside your own Exchange organization. They can be recipients on your Domino server or external recipients with a partner etc. A mail-enabled contact always has at least two addresses, but can have several. One or more are addresses from you own SMTP domain that let you see the contact in the Global Address List (GAL) and send mail to it. The other is the “real” address, namely the address that Exchange forwards the message to. This is usually an address outside your own SMTP domain.
On a mail-enabled contact you have two options for setting properties for the addresses; Set as Reply and Set as External. Set as Reply configures which address should be specified as the reply address when someone wants to reply to the message that was sent to the contact. This was known as the Primary SMTP address in Exchange 2000/2003. You can set one reply address for each address type, e.g. SMTP or Notes. Exchange requires that you have a primary, or reply address, for each address type.
Set as External is a little more interesting. It specifies where the message will end up; it’s final address. As mentioned above, a contact can have several addresses from you own SMTP domain. When a message is received by Exchange sent to one of these addresses, Exchange will look up the external address and forward the message to that address. In effect, the internal addresses are aliases for this external address. The address selected as the external address is stored in the targetAddress attribute in Active Directory. You can only have one address that is marked as external.
The addresses discussed here need not be SMTP addresses; both reply and external addresses can be either SMTP or another type.
Only contacts have the Set as External option, regular mailboxes do not need a target address since the message will be delivered directly to a store in your own Exchange organization.