When an SMTP session starts, xf-spf is expected to be forked from
qmail-smtpd - this is why you need SMTPEXTFORK. When forked, this processes
gets all enviroment variables which were available for the parent process 
(qmail-smtpd).

A customized SPF checker for qmail to be used with
SMTPEXTFORM patch, used and approved by a number of FreeBSD Brasil LTDA's
customers.

WWW:	http://www6.freebsdbrasil.com.br/~eksffa/l/dev/qmail-smtpextfork/