Thursday, July 9, 2015
Greylisting is the process of deferring emails from unknown senders. When the email arrives, Greylisting causes the server to return a message that boils down to, "I'm busy at the moment, try again in a bit." Valid Mail Transfer Agents (MTAs), like Exim, will automatically retry many times. This retry time can be several minutes to start and last for several days. Invalid MTAs will simply give up and move to the next enticing spam target. We use these retry attempts as a way to weed out good email from bad.