What is greylisting

Synopsis

This guide explains what greylisting is and how it is set at Sherweb.

Prerequisite

You must have an Exchange account with Sherweb

How to

What it is

Greylisting is a recognized and effective tool used to fight spam. It critically limits the amount of emails our servers accept from IPs that have not sent a consistent amount of legitimate mail to our users over a long period of time. Why? Because snowshoe spammers (a new spamming method) are vulnerable in one significant way: the IP addresses they send from only deliver very small volumes of email and never have a long history of sending legitimate email.

How it works

When an email is received from a server that has not contacted our servers in over a week, our anti-spam engine returns a temporary error (SMTP 4XX) asking the sending server to retry in 5 minutes. A properly configured server (following current RFC standards specifically sections 4.2.5 and 4.5.4.1) will respect this request and retry at least 5 minutes later, at which point our server will let the message through. If the server retries before the allowed time, it is blocked for another 5 minutes.

Note: The retry delay is configured on the sending server. This means if it is set to retry in 2hrs, the email will only be resent 2 hours later. Our own servers have no control over the sending server’s delay. Our servers only request to “retry in at least 5 minutes”.

Can I turn it off?

Yes. Please contact our support department to turn off greylisting for your account. Expect to receive more spam messages however.

Applicable to

All Sherweb Hosted Exchange accounts.