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+.TH s/qmail: qmail-postgrey 8
+.SH NAME
+qmail-postgrey \- send SMTP connection data to greylisting server
+.SH SYNOPSIS
+.B qmail-postgrey ip%netid;port Mail From: Rcpt To: TCPREMOTEIP TCPREMOTEHOST
+.SH DESCRIPTION
+.B qmail-postgrey
+is usually invoked by
+.B qmail-smtpd
+automatically provissioning the SMTP connection information
+.IR Mail\ From: ,
+.IR Rcpt\ To: ,
+.IR TCPREMOTEIP 
+and
+.I TCPREMOTEHOST
+to a greylising server given by
+.IR IPv4|IPv6%netid;port .
+.I port
+defaults to 
+.I 60000
+and thus can be omitted. 
+IPv6 LLU addresses can be specified
+adding the 
+.I netid 
+name following the percentage sign.
+.SH "GREYLISTING SERVER"
+Since there is neither a formal API defined for the
+greylisting lookup nor for the behavior and return
+codes of the greylisting server, 
+.B qmail-postgrey
+only works well with
+.I David\ Schweikert's
+.B postgrey
+implementation.
+
+Here, the server's response upon recognizing the triple
+.RI CLIENT_IP ,
+.I (SMTP\ envelope)\ SENDER
+and 
+.I (SMTP\ envelope)\ RECIPIENT
+is either
+.IR action=DUNNO ,
+.I action=PREPEND
+or
+.I action=DEFER_IF_PERMIT
+and in case of the last,
+.B qmail-postgrey
+returns with
+.I 10
+telling 
+.B qmail-smtpd
+to respond to the client with a SMTP
+.I 450\ greylisted
+reply code. Otherwise
+.B qmail-postgrey
+returns
+.IR 0 .
+.SH "INVOCATION"
+Unlike for testing reasons,
+.B qmail-postgrey
+is called directly from 
+.B qmail-smtpd
+in case the environment variable
+.I POSTGREY
+is defined and provissioned with the greylisting 
+server's IP address (and perhaps netid and port)
+listening there.
+
+The environment variable
+.I POSTGREY
+is typically defined within
+.B sslserver\'s
+.IR cdb .
+Additionally, 
+.I REPLY_GREYLISTED
+can be used as environment variable 
+to provide some more descriptive
+information to the sending MTA which will eventually
+be visible in a bounce message. 
+.SH "CREDITS"
+.B qmail-postgrey 
+and its integration into 
+.B qmail-smtpd
+is based on 
+.I Jan\ Mojzis
+implementation and used by permission.
+.SH "SEE ALSO"
+qmail-control(5),
+qmail-smtpd(8),
+https://postgrey.schweikert.ch