+ignores the line. However,
+.BR qmail-valid-addresses (8)
+interprets some markers in comments. If the first line of a
+.B .qmail
+file begins `#!' then the address (or addresses) controlled by the file
+are considered invalid, and any attempt to send to them is refused by
+.BR qmail-smtpd (8).
+(Note: This doesn't prevent local users sending mail directly. This
+feature can therefore be used to experiment with mail rules before
+allowing the entire Internet to break them.) If the first line of a
+.B .qmail
+file begins `#?' then
+.BR qmail-smtpd (8)
+will consult a Userv service
+.BI addrcheck: ext
+to decide whether addresses matched by the extension
+.I ext
+are valid. The service is passed many arguments, though the first two
+are the most interesting. The first is the local-part tail (i.e., the
+bit that matched
+.BR default )
+and the second is the envelope sender.