What is Greylisting and Will It Cut Down on My Junk?

Maybe you've just learned about blacklisting and whitelisting, and now they're talking about greylisting. What's the deal?

Black, white and greylisting are all techniques designed to cut down the spam that arrives in your mailbox. A blacklist contains addresses or domains from which you don't want to receive mail, regardless of the message contents. A whitelist, or trusted sender list, specifies those from whom you do want to receive. Whitelisting is particularly useful when your spam filters are putting legitimate mail into your junk folder.

These two methods have been used for years with varying degrees of success, but the spam problem is still going strong. Enter greylisting. This new technique is being implemented by ISPs in the hope that it will have a bigger impact on cutting spam than blacklisting has.

With greylisting, the mail server will temporarily reject an email coming from an unknown sender. Legitimate mail servers will then resend the message, which your mail server will now accept. Once the sender has been shown to be legitimate, that address will be added to the trusted sender list and there will be no delay for subsequent emails from that source.

Greylisting works because spam email tools generally don't bother resending rejected messages. For the spam that does get re-sent, often the delay between the initial transmission and the re-sent message will be enough time for more complex techniques to identify the message as spam and reject it on that basis.

The downside to greylisting is the delay of receipt for first-time senders, which may be anywhere from a few minutes to a few hours. This will be more noticeable with a service requiring you to confirm your email address before proceeding - creating a subscription on a website, or requesting a forgotten password. You will have a waiting period even if the website sends out your code immediately.

But with the promise of less spam, are you willing to accept a slightly longer delivery time for first-time senders? Only you can make that decision, and your email provider should be willing to give you that choice.

Learn more:
www.greylisting.org
Greylisting Whitepaper - by Evan Harris