Greylisting, or as some like to call it silverlisting, is the policy of asking unknown sending servers to retry later. Well managed email servers retry some random time later, while spamming servers move on and never return.
In this blogpost, Anthony Lieuallen of Brooklyn NY, writes about his experiences and statistics associated with greylisting. He describes the impact in this post too:
2,724 messages were blocked, 42 were passed, yet most were sent to bad addresses… hmmmm….
