I have heard from many sources that grey listing can be an effective weapon for fighting spam, yet hadn't had an opportunity to try it out.
However one of my clients was being hammered hard with spam, with 700+ messages a day being filtered by Intelligent Message Filter, and lots of messages getting past "I Hate Spam" from Sunbelt Software. Therefore I thought they would be a good site to test the method with.
I installed Vamsoft's ORF (http://www.shareit.com/product.html?productid=169362&affiliateid=200023740) on to the gateway machine and left it to get on with it, enabling just the grey listing and the automatic white list* feature.
* The automatic white list is built by watching outbound email and recording the email address used. When the external recipient replies, the message comes straight back in as the server then knows that the email address is legitimate.
The effect was immediate and noticeable. I watched the logs of the application very carefully to ensure that no legitimate email was being blocked. The amount of spam that was blocked by the application was considerable. After a running a week, the application reported that over 85% of all email that was being received was spam.
That doesn't count messages that were dropped by the filter on unknown users (http://www.amset.info/exchange/filter-unknown.asp).
The process isn't 100% effective, IMF was still catching some messages - but this was down to 20 or 30 a day, a massive reduction in the pre-grey listing number.
Users were also reporting that a few items were reaching their inboxes, but nothing like the level they had been receiving.
I have since deployed the application on four other sites, including my own Exchange server and seen similar significant drops in the number of spam messages being received.
As with user filtering, this technique also saves the bandwidth, as the messages are not even delivered to your server, so don't have to be processed.
The Vamsoft product works with any IIS SMTP mail server, so if you have Exchange 2000 then you can use it as well. It also features Active Directory filtering, which Exchange 2003 has built in, allowing users of the older version of Exchange to benefit.
How Does It Work?
Grey Listing is very simple idea.
A server attempts to deliver the message to the server. If the server hasn't received an email from that sender before, then it rejects the message with a temporary failure.
The systems that spammers use don't care about failure messages. They aren't interested in the failure and will therefore not try again. Spammers want to drop and run, before any system blocks the IP address that they are sending their email messages from.
However a legitimate email server will try again. Most email servers will try again for up to 48 hours, so you will get the email message eventually.
Are there any risks?
Any anti spam technique comes with risks. Unless you have a human looking at every message, you are relying on the computer making the decision whether the message is spam or not.
This technique will introduce a delay for new email messages - I have seen the delay as short as 90 seconds up to 20 minutes or more. If your business cannot tolerate any delay in email message delivery then this technique is not for you.
I have also seen a few email messages fail to be delivered from some sites that generate large amounts of email - such as eBay and a few ISPs. This is because each message appears to come from a different IP address in their server cluster.
With eBay, white listing their domain isn't advised as that will also allow in phishing emails.
While spammers don't comply with the RFC on SMTP email delivery and try just the once to deliver their email messages, this technique will be an effective first strike weapon in the war on spam. It shouldn't be considered the only weapon, but combined with other techniques can make spam more manageable.