sendmail modifies sender address

[geoff at hughes.net]

This problem is new to Fedora 8.  I'm running the (current) version, 
sendmail-8.14.1-4.2.fc8. Everything else is up-to-date. Sendmail 
configuration is unchanged from the disto.

My "From" address is being re-written by sendmail. geoff at mtranch.
mtranch.com becomes geoff at mtranch.mtranch.comQW. Internally (/bin/mail, 
balsa) this works fine, but my ISP (hughes.net) rejects it. 

Nov 22 09:41:46 mtranch sendmail[5424]: lAMHfb95005418: 
to=<geoff at hughes.net>, ctladdr=<geoff at mtranch.mtranch.comQW> (500/500), 
delay=00:00:07, xdelay=00:00:07, mailer=esmtp, pri=120344, 
relay=inbound.hughes.net...rehostedemail.com. [64.97.204.10], dsn=4.
0.0, stat=Deferred: 450 4.1.8 <geoff at mtranch.mtranch.comQW>: Sender 
address rejected: Domain not found

Full disclosure: mtranch.com is a bogus domain, but it has not been a 
problem in the past.

ISuggestions?