I found this on wiki.ctyme.com:
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Qmail is a popular MTA (Mail Transfer Agent) but it has a serious
flaw where is doesn't obey the rules when it comes to retrying other
servers when it receives a temporary error. Domains advertise multiple
MX records with priority numbers. The higher the number the lower the
priority (go figure). So mail servers are supposed to try the lowest MX
record first, then if that fails try the higher numbered MX servers.
Qmail is written so that if the lowest MX record returns a temporary
error then it refuses to try any of the other MX records and will
continue to talk to the same server till it times out and reject the
email. There is a patch available to fix this by the author of Qmail
refuses to include it in the distributed product. If you must use Qmail
we encourage you to use this patch. If you are selecting a MTA, Exim is
far better.
The problem this creates is that suspicious hosts are deferred on
our lowest MX but accepted on the second MX. So huge amounts of spam are
turned away while good email is only temporarily rejected but will be
accepted if the retry on the second level that has different rules.
Qmail doesn't return and therefore isn't compatible with this system.
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Is it just me, or is there something wrong with this logic? When
they say "mail servers are supposed to try the lowest MX record first,
then if that fails try the higher numbered MX servers," do they mean
server down so try the next one, or do they mean the email didn't pass
the mail filters on the priority mail server so try the next one? If
that's the case, why would you implement one set of rules on one server
and a different set of rules on the backup server (besides resource
limitations, which would say a lot about this hosting company)?
Is anybody familiar with this patch or why one would apply it?
-John B
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