And for those like me who shiver at the thought of Sendmail's m4 files, just edit /private/etc/mail/sendmail.cf with pico, find the lines containing:
# "Smart" relay host (may be null)
DS
and replace the DS line with:
DSoutgoing.mail.host.com
After restarting sendmail it should now relay all messages through this smart host. Do a tail -f /var/log/mail.log in the terminal to see where they end up.