my humble opinion, there is no reason for greylisting once you have postscreen (postfix native) enforced with DNSBL and DNSWL. Works awesome, fights 99% spam with few lines of config code adjusted.
Greylisting prevents delivery of email that predominately come from mail().
Greylisting works by waiting for a webserver to retransmit an email when it is refused entry. real email servers will generally retry over a period of 24 hours, scripts using mail() will not. A good majority of spam comes from PHP scripts.
This also explains why it targets a particular type of SPAM.
That is not my understanding of the postfix greylist milter, which I guess we are talking about. It is very simple to wrap a script around mail to periodically to test the same mail server until it’s accepted. Or are you refering to the php mail function ?
It might be simple but would a spammer do this (the majority of spammers use a scattergun approach → hit thousands of suspected recipients in the hope that a few are suckered in. They don’t want to waste their resources by repeating the same spam mail repetitively on one target)