Sendmail trably

http://www.sendmail.org/m4/anti-spam.htm...:
" ... As of 8.9, sendmail will refuse mail if MAIL FROM: parameter
has an unresolvable domain (i.e., one that DNS, your local
name service ...).
If you want to continue to accept such domains,
e.g. because you are inside a firewall, use
FEATURE (accept_unresolvable_domains)
..."
А еще лучше, пропиши у провайдера свой почтовый сервер
или поставь bind, если не лень :-)
Тогда эта фича не нужна будет