I got issues with both Mdaemon (multiple versions treating 4xx errors at
DATA as permanent errors) and with 3 servers running MS exchange 2003
(hiding messages from the queue and not retrying them until the service
was restarted).
I must admit it is quite hard to prove it as it is very hard to notice,
especially in the MS case as mails are not shown anymore in the queue
and exchange is not known for having some kind of useful logs. Also, it
is not always easy to get someone on the other side of the phone and ask
them to do some tests on their server when you are not managing it and
while they think *you* have problems, not them as they don't have
anything in their queue.
If you really wish some hard proof, I will have to install an MS
exchange server, although, as I said, exchange hides the mail in the
queue and doesn't log the failure, so I don't know what you would be
able to see.
I manage about 30 mail servers, all using greylisting for years (not
OpenBSD spamd, but a version running in the MTA). But as I greylist at
RCPT TO, I only noticed the problem it when clamav did go down and the
server was producing a 4xx error at DATA when it should have scanned the
mail.
Also, as an idea, I found it quite useful to whitelist only with a
triplet (from, to, IP), and not just the IP. Why? Because some people
are behind a firewall which allows them to go out with the same IP as
their mail server (yes, IPs are expensive in Europe), so windows
spamware is going out with the same IP than their mailserver and so
bypasses the filter.
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