A local domain is a Domain Name System (DNS) domain that is serviced by the local SMTP server. Any message that has a local domain name that arrives at an SMTP server must be delivered locally to a Drop directory or returned to the sender with a non-delivery report (NDR). Local domains are also known as service domains or supported domains. E-mail addresses that have local domain names are frequently known as local addresses.

If the domain is local, you can designate it as default or alias. There is one default domain. It is used to stamp message headers that do not have a domain specification. An alias domain is an alias of the default domain. If you add a domain and assign it as the new default, the previous default changes to an alias domain.

Domains that are not local are known as remote or nonlocal domains, and e-mail addresses that have remote domain names are known as nonlocal addresses. SMTP Server looks up remote domains in DNS.

If you want to set unique delivery requirements for a specific remote domain, you can add a remote domain and configure it accordingly. For example, you can add a remote domain and require that the SMTP service always uses Transport Layer Security (TLS) encryption in sessions with that domain. Or, you can change the routing so that messages sent to one remote domain are routed to another remote domain. Use the Domain Properties dialog box to configure domains.