John Hinton wrote:

I don't think my users would be too happy with greylisting, unless it was done only on blocklist, as they have come to enjoy the immediate delivery of email. Also, greylisting has the potential of hurting other ISPs, clogging their systems, just because they signed up a few 'stupid users' who got the latest virus/trojan. If you think back to some of the more successful viruses, mailservers everywhere suffered with many choking and going down. Adding to their mail queues isn't so nice.

While I have had a couple of mailservers that were sending legitimate mail complain about this (greylisting all mail), the vast majority have had no problem with it. I use the postgrey script (has it's own yum repo too :) and after a 5 minute delay the first time a triplet (client/sender/recipient) is seen it is auto-whitelisted. And the greylisting happens after all sanity checks and rbls. Vastly reduced spam from spambots which tend to just blast the mail out with no concern for the response. But I only have 200+ mailboxes and around 15-20k emails a day, so YMMV. I think the bottom line is that you have to pick your MTA/Content filter and then get on the mailing list and pay attention. It's an on-going war and there is no set-it and forget-it.

alex