Marc Perkel wrote:
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.
Having seen it before, I refer to this trick as the "I don't ever want
my e-mail trick". As you've discovered, that's the end result you get
from applying it. So long as you only want to get mail from servers
that mimic Sendmail's MX processing, it works. Otherwise, if the lowest
distance MX responds and tempfails, in essence saying "come back later",
then that's what qmail does. It's a feature, rather than a bug. If
that MX were not to respond at all, then qmail would contact the next
lowest distance MX.
- Ron
(who loved his MarxMenu to pieces, back in the day)