What are Relay Domains in Exchange Server 2007

Accepted or Authoritative domain is SMTP namespace for which Exchange Server 2007 is responsible for and for which you can send and receive e-mail. Accepted domain is automatically created when you first install the Hub Transport server. The authoritative domain is created with the internal domain name, but not with external SMTP name space. For example, if your Active Directory name is example.dom and registered SMTP domain name is example.com, then you will have to create an additional accepted authoritative SMTP domain for the registered SMTP domain.

Note: Accepted domain is not installed when you have an Exchange Server 2007 with the Edge Transport role.

Internal Relay domain is configured when your organization has two different forests and wants e-mails to be forwarded between these two forests. All internet e-mail destined  to second forest will be first received and processed by the Edge Transport server and then relayed to the Hub-Transport server in the same Exchange organization. The Hub Transport in your Exchange organization routes the messages to the Hub Transport server in the other Exchange organization. Just create a Send Connector from the receiving Exchange organization to the destination Exchange organization and an accepted domain from Internal Relay type.

External Relay domain is configured when you have an Exchange Server outside your Exchange organization. All messages destined for external domain are relayed through an Exchange Server 2007 Edge Transport server. MX Record is configured for external domain and SMTP Send Connector is used to route the messages, The send Connector relays the messages to the External relay domain.