Greylisting is a method of Spam filtering which is a cross between "Black Listing" and "White Listing".
It works by looking for the following information in an email:
1. The IP address of the host attempting the delivery.
2. The envelope sender address
3. The envelope recipient address.
From this we can then identify the email as it comes into our servers, this is called a triplet.
When we receive an email for the first time we make a note of the triplet information in our database (whitelisting the email) and then we refuse delivery of the email.
Because of the the way that the SMTP RFC has been implemented, the sending system will interpret this as a temporary failure to deliver the email and will try to redeliver the email.
When we see the email a second time we will then deliver it to the destination email address.
When a given triplet is first seen, it will be whitelisted in our database 10 minutes after first being seen, this is then sent out to all our mail servers which can take a further 5-10 minutes to finish synchronising.
If no mail has been accepted on a given triplet within 24 hours after being whitelisted, the entry is removed, meaning the triplet goes back to the initial "greylisted" state.
If mail is accepted, the entry is updated accordingly, and will be removed 7 days after the LAST mail was received from a given correspondent.
So if you receive mail from a given triplet at least once a week, the entry should never be removed.
Because most Spam programs do not act in the same way as a true mail server does, (they send the email and do not respond to any error messages), then this method will stop the majority of Spam emails sent from these types of programs.
This method has also been shown to be very effective against Email Viruses because the senders tend not to respond either.
Why have I been greylisted?
If you received a mail delivery report asking you to read this page, your mail server may not be compliant with the relevant Internet standards, and we suggest you talk to your technical staff or service provider.
The person you sent an email to uses Gradwell's "greylisting" service to filter their email. This service works by temporarily rejecting an email the first time it is offered for delivery by the sender's mail system. This results in a large decrease in received spam, because most spammers use simple mail software which does not correctly retry.
Unfortunately, if you are reading this page your mail server must also not retry properly.
All mail servers should retry a mail after a temporary "4xx" error code, such as the one returned for a greylisted address.
When you server retries more than 10 minutes after first sending the mail, our servers will accept and deliver the mail.
If you receive a "permanent error" or "delivery failed" message, it would indicate that your server is not retrying correctly, and we suggest you ask your technical staff or service provider to investigate the issue.
Alternatively, you may like to contact your correspondent via alternative means.
There is nothing we can do at Gradwell to assist you, this is a filtering mechanism your correspondent has chosen to use.