Milter-greylist

> Very few spammers still used dedicated systems to send out their wares, most now use bot farms.
>
> This means that they will be sending via real MTAs, which will retry.
>
> Looking at my logs over the last two days I see this:
>
> # grep "Greylisting in action" mail | wc -l
> 1234
> # grep "delayed for" mail | wc -l
> 1234
>
> So every email delayed by greylisting was eventually re-sent and accepted.
>
> Most SPAM was blocked by RBLs, that which passed (the 1234 above) then went on to SpamAssassin, which trashed around 1,210 of them as SPAM.
>
> Greylisting contributed ... zero.

you're misinterpreting the data.  the "delayed for" shows for every 
attempt.  try grepping for "autowhitelisted for" to see how many things 
retried successfully.