Starting with the Mail Server upgrade on 12/19/07, we have started using Greylisting as another tool to fight spam.
Greylisting temporarily rejects incoming email from a sender it does not recognize for 900 Seconds(15 minutes). Since temporary failures are built into the RFC specifications for email delivery, a legitimate mail server will attempt to connect again to deliver the email. The numbers of times and the time between retries is based on the settings of the senders mail server.
Once the initial 15 minute waiting period has passed, the sending server then has 360 minutes to retry sending the message. If the message is retried during that period, the email is accepted and the sender is added to the bypass list for 36 days. This means that there will be no blocks placed on the sending server for 36 days as a result of being greylisted.
What are the advantages of GreyListing?
We have seen the benefits of Greylisting in huge way. We estimate that Greylisting reduces spam received and processed by 80% or more.
Greylisting is effective because many mass email tools used by spammers will not retry a failed delivery. As a result, the spam is never delivered. When a spammer does retry a delivery after the waiting period has expired, it will likely be after a number of automated honeypots have detected the spam source and listed both the source and the particular message in their databases. Subsequent attempts by spammers are more likely to be detected as spam by other anti-spam mechanisms.
More information:
For more information and a more technical overview of Greylisting, we recommend reading http://en.wikipedia.org/wiki/Greylisting
