greylisting
DELIVTERMS: The weekly series here on Spam Resource that defines deliverability terminology. Today, I’m going to talk about Greylisting.What is it? Greylisting is where an ISP or mailbox provider configures their mail server to temporarily delay incoming email. Usually not long — perhaps for only 60 seconds to a few minutes.How? Mail servers that support this have a setting where they track server connections. If it’s the first time a certain server or domain connects to them, they defer (temporarily reject) that connection or address with a 4xx temporary failure. This invites the sending server to try again later. Most sending mail servers have a “retry interval” setting that governs how long until they retry. In some cases, it’s 15 minutes, in other cases, a few hours. This can usually be configured by the administrator of the sending MTA.Why? It’s a form of spam filtering. It is done to stop…