Now, where greylisting IS appropriate and very useful is on backup MX servers.
This does not consort with your observations. Because when a remote client will choke on a temporary deferral and will return the mail, it will also do so when it has to contact the backup MX (because the primary MX is down, and thats what the backup MX is for).
My experience with greylisting produced similar results. Broken MTA's bounced the email back to the sender immediately, or some would fail on their retries and send a "Couldn't get there for the past hour" message to the sender. Customers didn't like the delay, especially when it was first set up.
Are the MTA's that are broken enough to die on a 450 smart enough to try a backup MX? :)
I would have to agree with the idea that they're useful on the backup servers, at least during "normal" conditions (primary is up). When I had secondaries configured for some domains, they got more delivery attempts by spammers and viruses than the primaries did. I guess they figure most backup servers aren't configured as tightly as the main servers are, or aren't as busy.
However, in a situation where the main server is down and the backup servers are taking the load, I would probably back off the greylisting until the main server's back up.
-- Bryan
