The SMTP site has at least one domain¾the default local domain. You can add more domains and configure them to be local or remote. You can delete any domain except the default domain.
A local domain is a DNS domain that is serviced by the local SMTP server. Any message with a local domain name that arrives at an SMTP server must be delivered locally to a Drop directory or returned to the sender with an NDR. Local domains are sometimes referred to as service domains or supported domains. E-mail addresses with local domain names are often referred to 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 lack 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 non-local domains, and e-mail addresses with remote domain names are referred to as non-local addresses. Microsoft SMTP Service 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 Microsoft SMTP Service always use TLS when sending messages to the 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 property sheet to configure domains.