look at /home/vpopmail/bin directory. there are some binary files which can help u to check domains. also check the files in /var/qmail/control directory, especially rcpthosts file.
another option is reinstalling qmail. there is a great installation guide that includes antivirus and antispam at
www.qmailrocks.org
backup /home/vpopmail and /var/qmail directory. after installation copy back necessary directories or files.
it will work.
good luck