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FAQ for Greylisting

Welcome to The WELL! Here are the answers to some questions about Greylisting.


What is Greylisting?
What is tuple?
How does this stop spam?
What kind of results can I expect?
Will Greylisting delay legitimate email?
How long will Greylisting delay legitimate email?
Can I whitelist certain tuples, senders or IP addresses?
Will Greylisting affect email that I send?
How do I turn Greylisting on?


What is Greylisting

Greylisting is a clever anti-spam technique that introduces a short delay before accepting mail. When a message with a given "tuple" (see next question for definition) arrives, greylisting initially refuses to accept the message, failing with a temporary rejection code. If that same "tuple" is seen again after a suitable delay, the mail is then accepted.

What is "tuple"?

Every message that arrives comes from a specific server (identified by an IP number), and has a specific sender address, and a recipient address. These 3 pieces of information - the IP, the sender, and the recipient - comprise a tuple. The greylisting software uses this tuple to determine whether it should issue a temporary reject code or accept the message.

How does this stop spam?

Legitimate mail servers respond to a temporary rejection code by simply queueing the rejected message for later delivery. Such servers will attempt to deliver the message again at some time in the near future - typically after a delay of 5 minutes to an hour, but the interval can be shorter or longer depending on how the delivering server is configured. Some spam servers also work this way, but many do not - many spam servers don't bother to check whether delivery was accepted, so mail blocked with a temporary rejection code is simply never delivered.

What kind of results can I expect?

The actual amount of spam reduction you can expect to see depends a great deal on how much spam you receive, and where that spam comes from. Our tests show a wide range of results with some people experiencing only minor reductions in the amount of spam received, and others experiencing major reductions.

Will Greylisting delay my legitimate email?

Yes, it will. When mail with a given tuple arrives, the greylisting software checks its database to see if that tuple has been seen recently. There are 3 possible situations:

1. The tuple has not been seen at any time in the past 7 days.

In this case, the mail is temporarily rejected, and a 10 minute countdown timer for this tuple is started.

2. The tuple was first seen less than 10 minutes ago.

In this case, the mail is temporarily rejected again, but the 10 minute countdown timer is allowed to keep ticking.

3. The tuple was first seen more than 10 minutes ago, but less than 7 days ago.

The mail is accepted. Each time a message with the same tuple is seen thereafter, the 7 day countdown restarts.

How long will Greylisting delay my email?

At least 10 minutes for tuples that aren't currently in the 7 day "whitelisting" period that starts after a message is accept. In most cases, mail will be delivered within an hour, but depending on how the sending server is configured, it could be much longer.

Can I whitelist certain tuples, senders or IP addresses?

Not under normal conditions - greylisting is intended to be a low-maintenance low-resource anti-spam technique, and user configurable whitelisting would tend to be at cross-purposes with that.

Will greylisting affect email that I send out?

Mail that you send will not generally be subject to greylisting delays.

How do I turn on Greylisting?

- Go to:

- Select the Spam option

- Greylisting can be turned on in the Greylisting section of the page. You can also request a nightly report about how greylisting is working for you.

- Click the Update button

For Further Information...

For information about The WELL or for WELL Customer Support, use the Helpdesk contact information on the footer of this page.

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