The "mailertable" feature enables manual routing of mail. The /etc/mail/mailertable
file will contain instructions about how and where to forward mail for specific domains. The entry
simply routes mail for the "mydomain.com" domain to "mail.mydomain.com", overriding any MX records.
You could also forward mail to an SMTP service on a different host and/or port, or you could specify that mail should be transfered using another protocol, like UUCP.