A recipient in Microsoft Exchange Server that doesn't reside in the Exchange organization. When creating a custom recipient, you specify the e-mail address of the remote user first, and then configure the properties of the recipient. An example of a custom recipient is the Simple Mail Transfer Protocol (SMTP) address of a user on the Internet.
Custom recipients are often created on Exchange servers to place frequently used foreign addresses in the global address book so that users don’t have to specify the recipient's e-mail address manually or maintain their own personal address books.
Custom recipients can be used for various other purposes in Exchange, such as to enable a user's Internet mail to be forwarded to his or her Exchange mailbox.