Neat story, but in 2016 i'm still sort of surprised to see anyone pushing Greylisting as a solution.<p>I guess that it really depends heavily on your situation. For many setups, greylisting causes more delivery problems and help desk tickets and unhappy users. Especially when <i>so much email</i> these days is coming from huge server farms like office365 and especially gmail. With greylisting, I've had mails from gmail users be delayed over an hour before. Mails just kept coming from different SMTP hosts. I was sitting there watching the retries happening.<p>I had also noticed that over time, spam bots were retrying as well and more spam was slipping through. Although, by the time they got through, they had already been added to RBLs.<p>Like I said, it depends. After many years of having greylisting enabled, I finally removed it from the chain and went with Postscreen and so far (since Aug 1st) it's been just as effective for me. With none of the delivery issues that were happening before.<p>But your mileage may vary.