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HomeKnowledge BaseEmailWhat is Greylisting and how does it work?
Article ID124
Created On5/9/2008
What is Greylisting and how does it work?
*NOTE* - Greylisting is not enabled on your domain by default. See below on how to enable it.

What is Greylisting and how does it work?

Greylisting is a new tool in the fight against spam. What it does is temporarily block incoming mail from a sender. It returns the mail to the sender's mail server with a message saying try again later. The sending server must then retry sending the mail after the Block Period but before the Pass Period(see below for definitions of these values).

Greylisting is effective because spammers will not usually bother to attempt a second delivery, but legitimate e-mail servers will.

Why use Greylisting?

Greylisting is a very effective method of spam blocking that comes at a minimal price in terms of performance. Most of the actual processing that needs to be done for Greylisting takes place on the sender's server. It has been shown to block upwards of 95% of incoming spam simply because so many spammers don't use a standard mail server which will do automatic retries.

Disadvantages of Greylisting

The biggest disadvantage of greylisting is the delay of legitimate e-mail from servers not yet verified.

On the first attempt a sender's server connects to our servers, we respond with '4.5.1, please try again later' (This is acceptable according to applicable SMTP standards). We allow the server to reconnect in 2 minutes. On the second delivery attempt we allow the mail to be delivered and add the server to a "whitelist" for 12 hours, allow successful connections on the first attempt. The actual delay it dependent on how quickly the sending server reconnects.

How do I enable Greylisting?

To enabling greylisting on you domain, simply Click Here and send a support ticket requesting greylisting be enabled on your domain.