I'm not a Qmail user. I have a spam filtering operating where I do front
end filtering for about 3000 domains. Many of the servers that send mail
to my filtering network are running Qmail and there seems to be a
problem and I'm wondering if someone can address it. I'm running Exim
myself.
One of my tricks to filter spam is a gray listing like trick that
detects suspicious hosts and returns a temp error on the lowest MX
number. Spammers often don't retry but real email servers would, in
theory, retry the next level up in the MX chain and the secondary server
will accept the email.
Servers that I do this with include servers with no or bad reverse
lookup, Host names with pattens that look like residential machines, and
servers listed in black lists that are not reliable enough to block, but
usually are spammers.
The idea being the but profiling these servers and returning a temp
error (421) on the lower MX that the good servers who would be a false
positive would retry to a different server that would accept it.
But - it seems like servers who are running Qmail only send to the
lowest MX and don't retry the higher MX. Is this so? Or does it apply
only to old versions?
When Exim gets a temp error on the lowest MX it immediately retury all
the IP addresses of the higher MX servers. If they all fail then the
server wait for a period of time and tries them all in order again. But
people are telling me that Qmail is broken on this issue.
So - is this so? Can someone let me know how Qmail works on MX retries?
Thanks in Advance
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