Exchange Server 2007 introduced new terminology for describing the domain names that it will accept email for, and what it will do with that email. This is referred to in Exchange Server 2007 as Accepted Domains.
In Microsoft’s own words, “an accepted domain is any Simple Mail Transfer Protocol (SMTP) namespace for which a Microsoft Exchange organization sends or receives e-mail.”
Accepted Domains fall into one of 3 categories – Authoritative, Internal Relay, and External Relay. Any given namespace that is an Accepted Domain can be only one of those three types.
Authoritative Domains are those for which an Exchange organization hosts mailboxes that have email addresses that use that domain.
For example, a company named Contoso Pty Ltd may own the domain name contoso.com and use email addresses of firstname.lastname@example.org. The Exchange organization would be configured to consider contoso.com an Authoritative Domain.
An organization can have more than one Authoritative Domain configured. Using Contoso Pty Ltd as an example again, they may have a second brand name of Contoso Services and use the contososervices.com domain name in marketing materials. In this case the Exchange organization would be configured with both contoso.com and contososervices.com as Authoritative Domains.
Internal Relay Domains
Internal Relay domains are those for which an Exchange organization hosts some, but not all of the mailboxes that use that domain. This scenario is sometimes also referred to as a “shared SMTP namespace”.
Internal Relay domains are common when two companies have merged but are yet to consolidate their Exchange environment into a single organization. When they have a need for consistent email addressing across both Exchange environments Internal Relay domains are the solution.
When an Accepted Domain is configured as Internal Relay it tells the Exchange organization to accept mail for that domain, but if no recipient in that organization has that email address then it looks to the list of Send Connectors to determine where to send it next.
For example, if Contoso Pty Ltd and Northwind Traders formed a new company Contoso Traders with a new domain name of contosotraders.com, then each existing Exchange organization is configured with two items to share the SMTP namespace:
- An Internal Relay domain of contosotraders.com
- A Send Connector for the namespace contosotraders.com that sends email for unknown recipients to the other Exchange organization
External Relay Domains
External Relay domains are those for which an Exchange organization will accept email, but hosts no mailboxes for that domain. This scenario might occur when one organization is acting as an ISP for other organizations, or offering services such as email content filtering.
External Relay domains are used when one Exchange organization is accepting email from the internet for a non-authoritative domain name, and then forwarding it on to the authoritative Exchange organization. This is usually performed at the Edge Transport Server to keep email for non-authoritative domains from entering the corporate network. For this to occur the Edge Transport Server is configured with two items:
- An External Relay domain
- A Send Connector for the namespace that sends the emails to the authoritative Exchange organization
In these scenarios it is also common for the Edge Transport Server to be used as an outbound email relay, or smart host, for the authoritative Exchange organization.
For most Exchange organizations the Authoritative Domain type is the only one used, however it is important for email administrators to understand the full capabilities of Accepted Domains as explained above.