What made sendmail do this?

[Adam Myrow]

I've been using a combination of Fetchmail and Sendmail to send and
receive email with no problems every since I first started using Linux.
Today, however, something weird happened that has never happened to me
before.  Sendmail suddenly started rejecting all incoming email with no
explanation.  It started saying that the domain does not resolve.  For
example:
 Jun 15 17:23:15 homerun sendmail[1373]: g5FMNFS01373:
from=<speakup-admin at braille.uwo.ca>, size=3406, class=0, nrcpts=0,
proto=ESMTP, daemon=MTA, relay=myrow at localhost [127.0.0.1] Jun 15 17:23:16
homerun sendmail[1374]: g5FMNGS01374: ruleset=check_mail,
arg1=<speakup-admin at braille.uwo.ca>, relay=myrow at localhost [127.0.0.1],
reject=451 4.1.8 Domain of sender address speakup-admin at braille.uwo.ca
does not resolve
It may have caused Fetchmail to delete some messages, but not all of them.
Anybody know what could have caused this?  I don't remember changing
anything.  For now, I have Pine using my ISP's Imap server and SMTP server
directly, but this is slow over a dial up which is why I have always used
Fetchmail and Sendmail.  Any suggestions welcome.