Accepted domains are the SMTP name spaces (also known as address spaces) that you configure in an Exchange organization to receive email messages. You use the Exchange admin center (EAC) or the Exchange Management Shell to configure accepted domains in Exchange Server.
For more information about accepted domains, see Accepted domains in Exchange Server. The types of accepted domains are summarized in the following list:
Authoritative domains
All recipients in the authoritative domain exist in the Exchange organization.
Exchange is responsible for generating non-delivery reports (also known as NDRs or bounce messages) for non-existent recipients in an authoritative domain.
Internal relay domains
Some recipients in the internal relay domain might exist in the Exchange organization.
Exchange isn’t responsible for generating NDRs for non-existent recipients in an internal relay domain. Instead, you create a Send connector with the address space of the internal relay domain. You source this Send connector on an internal Mailbox server to relay messages for the non-existent recipients in the domain.
External relay domains
None of the recipients in the external relay domain exist in the Exchange organization.
Exchange isn’t responsible for generating NDRs for non-existent recipients in an external relay domain. Instead, you create a Send connector with the address space of the external relay domain. You source this Send connector on an Edge Transport server or Internet-facing Mailbox server to relay messages for all the recipients in the domain.
Authoritative domains: Recipients (in particular, mailboxes) are configured with email addresses in these domains. The Exchange organization accepts messages that are addressed to recipients in these domains, and is responsible for generating non-delivery reports (also known as NDRs or bounce messages) for non-existent recipients.
Relay domains: The Exchange organization accepts messages that are addressed to recipients in relay domains, but isn’t responsible for generating NDRs for non-existent recipients. Instead, Exchange (with additional configuration) relays the messages to messaging servers that are external to the Exchange organization. Relay domains can be internal (for domains that you control) or external (for domains that you don’t control).