SymptomsIf a email message could not be delivered (domain does not resolves or anything else), the message is being returned back to sender with am empty "FROM" field.
So, if the reason of the mail flow failure is the unknown recipient in the target domain, the qmail will send a message to local address of postmaster (usually "postmaster@localhost") with the "#@" content of the "FROM" field.
ResolutionThere is a hot-fix for CentOS release 5.2 (Final) both 32 and 64 bit. Other OSes were not tested with the solution.
To apply the patch replace the /var/qmail/bin/qmail-queue file with the attachment (x32 or x64).
Read the http://kb.sp.parallels.com/en/8083 article about a hot-fix installation procedure.