In short, this is a method to block spam by a) Read and Store SenderIP/SenderAddress/RecipientAddress triplet b) Deny mail with a temporary SMTP error (451 Try later) c) Accept same mail when it is retrying after a specified waiting period d) Accept same triplets for a specified time
This is how postgrey implementation for postfix works in advanced IMGate. I've installed postgrey on several sites, including high-volumes sites, and it works extremely well.
For the first 6 hours of Tue:
========================= Postgrey Greylist Rejects ========================= Total: 18846 Net : 15303
"total" is new triplets rejected (never seen before, so not cached in the postgrey database).
"net" is new triplets greylist-rejected that have not been re-tried (so far) and are effective rejects.
(total - net) = 3500 are msgs that were 450-rejected, and then re-tried and accepted by greylisting (but perhaps rejected by a filter running after greylisting).
The ratio of the numbers above holds pretty solid for this site, with the total postgrey-rejects running up 120K/day.
As you pointed out, some spam farms do not retry after a 4xx reject.
But ime, the largest source of greylist rejects is the millions of compromised/RATware/infected PCs on notorious subscriber networks, un-policed by the network operators, doing direct-to-MX spammings/virus-pushing. Not being full, SMTP-compliant MTAs, they have no queue/defer/retry mechanisms, so the 450 greylist reject is perfectly effective against them.
Since Imail doesn't do greylisting, and Declude can't do greylisting, ignore the smearing, self-serving FUD from Scott about greylisting, as if one implementer's reputation had anything to do with the technique you describe, which probably the most common greylisting technique.
Len
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