It's been around for a while now but people still ask me about Greylisting most days. So, what is Greylisting? The official wikipedia description of Greylisting tells us that ... "Greylisting (or graylisting) is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again and the email is accepted. If the mail is from a spammer it will probably not be retried since a spammer goes through thousands of email addresses and cannot afford the time delay to retry."
What does this mean to you as an email receiver? It means that email servers which utilise Greylisting do so in an attempt to block most SPAM email at the Server level. This bounce and resend procedure affects email delivery which now will be delayed sometimes by 20 minutes or longer. Welcome to the new age of email, courtesy of the SPAM'ers.
It's important to note that Greylisting does not automatically bounce all email, just email from unknown senders.
Whitelisting and Blacklisting
So, if we're going to discuss Greylisting we need to touch on Whitelisting and Blacklisting because Greylisting sits somewhere in the middle of these two.
Your email provider should give you access to both Whitelisting (Trusted Senders) and Blacklisting (Banned Senders) of your emails. In other words, the ability to sift through your email at the server-side level and assign either Whitelisting or Blacklisting to individual emails you receive.
Educating your email server at this level is to your own advantage because it means email from "trusted senders" in your Whitelist will by-pass Greylisting and be delivered to your inbox, providing they pass the server-side and client-side virus-scanners as well of course (another story entirely). And, similarly, email from "Banned Senders" in your Blacklist will automatically be dumped into your server-side "junk folder". A great solution for most of the email SPAM trash floating around out there at the moment.
Delayed Email Aggression Syndrome
Most people are used to automatic delivery of email and because of the delays incurred with Greylisting the dreaded DEAS occurs (Delayed Email Aggression Syndrome). DEAS is something I have just made up but it aptly describes the reaction by most consumers to the knowledge that their mail is being delayed.
Another cause of DEAS is mail that is NEVER delivered. Why does this happen? It's because some ISP's don't play fair and use different IP addresses on their SMTP servers so Greylisting doesn't work as the IP address can be different each time an email is sent from this ISP to a Greylisting type server. In a nutshell this means that the Greylisting server will say to an unknown sender " try again later " and the ISP will resend but from another IP address which the Greylisting server doesn't recognise and will treat as an email from an unknown sender and say " try again later ". In effect creating a loop and the email will never be delivered. The only way around this scenario is to Whitelist the ISP's sending server(s) IP address(es).
What can you do?