Why was mail sent to me returned to the sender with a notice saying they were “greylisted”?
The short version: The sender’s mail server is not behaving according to the standards, in particular the rule that says it should try again later if it receives a temporary deferral from our server. (Yes, there are more of these than you would expect, even at big email providers who should know better.) Their email provider should fix the problem, but we can also work around it by whitelisting if necessary. The long version: Greylisting is our most effective anti-spam measure, and it works by giving a temporary failure (a 4.x response) the first time a sending server tries a message from a new sender to a new recipient. All legitimate mail servers should handle this temporary failure by deferring the message and trying again later, often within 15-30 minutes, at which point we accept the message and whitelist the triplicate of sending server, sender, and recipient, so mail will flow unimpeded on future attempts. (Mail servers are also fully whitelisted after a certain number of succes
