To edit a mail domain, click Edit under Actions column.

Authoritative: An accepted domain is authoritative if email is to be delivered to recipients only in this Exchange organization.

Internal Relay: An accepted domain is internal relay if email is to be delivered to recipients in this Exchange organization or relayed to an SMTP server outside this Exchange organization. Keep this setting if the domain is shared by this Exchange organization and another messaging system.

External Relay: An accepted domain is external relay if email is to be relayed to an email server outside this Exchange organization.