Virtual Hosting with Sendmail

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, virtual users (virtuser@virtdomain.com) can send without a problem!
Have rechecked all my Sendmail config, dns records, all in vain. When sending a message to the virtual user, if you cc or bcc virtuser@localdomain.com, virtuser@virtdomain.com receives the message but if you just send to virtuser @virtdomain, you receive the error reported above. Comment are welcome!