is only consulted if the IP address of the connecting client does not have a PTR record in DNS. In that case tcpserver doesn't set TCPREMOTEHOST and qmail-smtpd sets the variable remotehost to unknown. If this is true qmail-smtpd with the diff applied does an additional check of the domain of the envelope sender against the control/badmailfrom-unknown control file. The syntax for that file is the same as for control/badmailfrom.
The benefit of this modification is that a lot of spammers use evelope sender addresses of wellknown public mail services like
The patch is "shielded" by the #define __mBMF_UNKNOWN at the top of qmail-smtpd.c. Undefining it disables the modification.
Within one week running qmail-smtpd with this modification we filtered out about 1200 messages a day on a system receiving about 80000 messages a day.