Sendmail virtual hosting (on RH 8.0)

Had configured successfully Sendmail to accept mail for several domains. Worked properly till recently. All the sudden, mail sent to the hosted domains returns the following error to the sender:” Unable to deliver message to the following address(es).
:
Sorry, I couldn’t find a mail exchanger or IP address. (#5.4.4)”. However,users of the virtual domains (virtuser) can send without a problem!
Have rechecked all my Sendmail config, dns records, all in vain. Any suggestion? Thanks.

Angelo.