baffled by sudden problem

I have a Majordomo installation on a server local to my domain that 
has been working fine for many years.  Without making any changes, 
suddenly posts to the mailing list that originate inside my domain 
still work fine, but posts that come from outside my domain no longer 
make it out to the list.  As near as I can tell from the maillog 
logs, Majordomo never even gets its hand on the incoming message 
destined for the list - I never see the initial "clone" that 
Majordomo always does.

I tried rebuilding my aliases and my sendmail configuration.  No joy.

Anybody have any ideas where to look?  What log files?  I am really 
stumped on this one.

Thanks,
Howard