The standard sendmail.cf file uses mail domains to determine whether mail is delivered directly or through a mail host. Intradomain mail is delivered through a direct SMTP connection, while interdomain mail is forwarded to a mail host.
In a secure network, only a few selected hosts are authorized to generate packets that are targeted to external destinations. Even if a host has the IP address of the remote host that is external to the mail domain, the establishment of an SMTP connection is not guaranteed. The standard sendmail.cf assumes the following.
The current host is not authorized to send packets directly to a host outside the mail domain.
The mail host is capable of forwarding the mail to an authorized host that can transmit packets directly to an external host. Actually, the mail host can itself be an authorized host.
With these assumptions, the mail host is responsible for delivering or forwarding interdomain mail.