In case anyone is/was trying to use exmh or some other mailer that uses MH,
and is/was getting as frustrated as I was failing to 'send' mail, I have been
pointed toward the solution by Marc Mengel:
In the file /usr/lib/mh/mtstailor, the last line looks like this:
servers: localhost \01localnet
However, sendmail on the local machine is configured in such a way that it
will not relay mail (a rudimentary method to thwart spammers from using your
machine as a relay point).
This line should like this:
servers: fnal.fnal.gov \01localnet
Since fnal.fnal.gov is the lab mail server, and is configured to relay mail.
Cheers,
Dan