Hello,
The first thing is to determine the origin of your problem. The most usual way to reject incomming mail is to use access.db feature, so I think you should check if this is what actually blocks the mail comming from that domain.
Usually there's a "database" file called /etc/mail/access.db which is built from a human readable file, probably /etc/mail/access.
Check if this is what limits the incomming mail, and if so, edit the file and rebuild the database.
Look for the documentation at sendmail.org to learn the sintax, or just post again for more info.
Bye,
jmiturbe