Reducing Spam - Grey-Listing
Grey-listing is a new weapon to use against spam in this great war being waged upon it. With this new shielding method, by which you may block out huge amounts of spam, you are sure to please your email clients!
In name, as well as operation, grey-listing is related to white-listing and blacklisting. What happen is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a "try again later"-message (This happens at the SMTP layer and is transparent to the end user). This, in the short run, means that all mail gets delayed at least until the sender tries again - but this is where spam loses out! Most spam is not sent out using RFC compliant MTAs; the spamming software will not try again later.
If you want to look in more details at what is involved look at this wikipedia entry: http://en.wikipedia.org/wiki/Greylisting
We have been running GreyListing for about 2 years now and it has cut down the amount of spam hitting our machine drastically.
This is a bit technical and we would recommend you get support to set this up for you. The brief steps for those that want to do it themselves are in the file
/usr/utilities/GreyList.txton your machine.
Once enabled the software will just work away in the background. You can look in the security report for MX Deferral entries - this is the number of messages that have been delayed.
There are a couple of things to keep in mind:
- You might get a false positive - there may be the odd site who's mail setup is non-compliant and it is rejected.
- You may not like the 'try again later' delay.
We would also recommend when you initially enable GreyListing you add your 'common' domains to the whitelist - use the last couple of security report and get a note of the 'top' addresses you receive mail from.
Use the Admin login,
Enter all the top domains (if you have a lot please see support for a short cut for initially populating the list) - now these sites will be passed straight through.
In practice, we have not really been bothered with the delay, yes on a couple of the sites where they will E-Mail you confirmation link before you can proceed it's a bit annoying. However the reduction in Spam is worth it.
Monitor the mail logs for the 1st few weeks - once you happy just leave it be and it will maintain it'self.
There are other options accessible on the Admin login relating to Grey Listing - you can add and delete senders and add and delete sites to the blacklist.