Badmailto
From Qmailtoaster
badmailto
Note, badmailto applies to both incoming and outgoing messages.
man qmail-smtpd excerpt:
Unacceptable envelope recipient addresses. qmail-smtpd will reject every recipient address for a message if the recipient address is listed in, or matches a POSIX regular expression pattern listed in, badmailto. For more information, please have a look at README.qregex.