Your SMTP Relay server is promiscuous because you've misconfigured the smtp-auth extension to qmail.

You've come to this page because you've asked why the senders of Unsolicited Bulk Mail are able to relay their mail through your SMTP Relay server, and, upon testing, the dialogue with your SMTP Relay server has looked something like this:

334 VXNlcm5hbWU6
334 UGFzc3dvcmQ6
235 go ahead

This is the Frequently Given Answer to that question.

You are employing an smtp-auth extension to stock qmail of some sort, but you haven't configured it in the manner that is described in its documentation. As a consequence, your SMTP Relay server is promiscuous.

In the case of Krzysztof Dabrowski's smtp-auth patch, you have forgotten the required hostname argument to qmail-smtpd. This means that instead of invoking qmail-smtpd as

qmail-smtpd hostname /bin/checkpassword /bin/true
you are invoking it as
qmail-smtpd /bin/checkpassword /bin/true
and as a consequence qmail-smtpd is taking your hostname to be "/bin/checkpassword" and your password checking program to be "/bin/true". Because /bin/true always exits with a zero status, you are telling qmail-smtpd that all user+password combinations are succeeding.

The senders of Unsolicited Bulk Mail have taken to scanning for SMTP Relay servers that allow any user+password combination, and using them to relay unsolicited bulk mail.

© Copyright 2003 Jonathan de Boyne Pollard. "Moral" rights asserted.
Permission is hereby granted to copy and to distribute this web page in its original, unmodified form as long as its last modification datestamp is preserved.