Greylisting is a term used to defend or to block spam at the sender server. It blocks the spam emails based on the behaviour of the sender server instead of content and images of the email campaign. This term is generally used by URIBL and they maintain a list of greylists, whitelists and blacklists. Procedure to get into a greylist:URIBL say the following on their website - "Greylist is a list which contains domains found in UBE/UCE, and possibly honour opt-out requests. It may include ESPs w...