what is greylisting?
Posted: 09 Dec 2016 18:31
What is greylisting and why does it take so long time to deliver mail that is in waiting?
https://en.wikipedia.org/wiki/Greylisting
In short words, it blocks all first mail delivery attempts from an IP, and allows the second attempt after 5 minutes.
Most spammers that use botnets use a single delivery attempt, thus making it an effective way of filtering spam. but with the drawback that all mail is delayed with at least 5 min.