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Applied Innovations uses SmarterMail 4.3 for our windows shared hosting mail servers, Windows dedicated servers and Windows 2003 VPS Servers. One of the reasons we moved to SmarterMail 4.X was the promise of better spam fighting techniques and that Greylisting alone could deliver a reduction of over 80% in total spam making it through normal spam blocking measures (keyword filters, RBLs, blacklists, etc).

What is Greylisting?

The quick explanation of greylisting is: Each time a message is received if it’s the first message your mail server has received from the sender’s mail server, the mail server says “Sorry I can’t take this message, I’m busy. Please try again in a few minutes”. A normal mail server will wait a few minutes and then try the message again at which time your mail server will say “Oh Hi! It’s you again, sure come on in.”  and the message will be delivered.    Then after that for a period of time any more messages from that same sending server will be delivered without the delay.  

This simple technique of delaying messages believe it or not is currently an extremely effective anti-spam technique.  The reason it’s effective is that spammers try to send millions of messages as quickly as possible. If they get a “sorry, I’m busy” message they simply move on to the next email address in their list of messages.  They don’t have time to worry about retries because they’re sending so many messages and they really don’t care.   Whereas a real mail server does care about retries and will retry the message.  So that’s the secret sauce to greylisting.  You can get more information on greylisting from Wikipedia.

So my messages get delayed, why all the hooplah?

Yeah it’s a pretty basic concept isn’t it. It’s pretty much the same technique you’ve been doing to telemarketers for ages “Sorry he’s not here, please call back later”. But does it work?  Well, here’s some stats from a couple of our mail servers that should answer that question:

Server #1


Server #2


That’s just two of the mail servers we operate for our shared hosting servers but between those two mail servers more than 1.5 million spam messages were blocked. So yes, it works.

There’s no such thing as a free meal, right?

Yes it’s an old cliche. Of course, if you prefer a little more modern of a saying how about “With great power, comes great responsibility”.  Greylisting does come with it’s negatives.

  1. For one when that first message from that one mail server comes through, it’s going to be delayed and you *might* notice the delay. For very busy mail servers though and people you interact via email often this is not a problem. Plus if you have that one user that’s time sensitive you can disable greylisting for their account or for the entire domain if you wish. 
  2. Eventually spammers will figure out how to get around this but don’t worry the SmarterGuys from SmarterTools are already working on this (and we have a couple other surprises in the works already ourselves).
  3. Mail from a server that doesn’t support retries may not get delivered. All of the web servers we operate are whitelisted and this hasn’t caused a problem for most of our customers (I’d say under a dozen have had an issue with greylisting out of 10,000+ and we’ve disabled it for those customers).

When it’s said and done, Greylisting Rocks!

Yes, that’s right, Greylisting ROCKS!.  Greylisting has proven to be a very effective tool against spam at with very little impact on normal day to day email activities.

By implementing greylisting many of our clients have reported a 80% to 90% drop in spam and the numbers above are testament to those claims.

If your host doesn’t support greylisting and you’re just fed up with all the spam, I encourage you to checkout one of our shared hosting promotions going on right now and start benefiting from the reduced spam.

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About Jess Coburn

It's Jess's responsibility as CEO and Founder of Applied Innovations to set the direction of Applied Innovations services to ensure that as a company we're consistently meeting the needs of our customers to help drive their success. In his spare time, Jess enjoys many of the things that made him a geek to begin with. That includes sexy new hardware, learning new technology and even a videogame or two! When you can’t find him at the office (which admittedly is rare), you’ll likely find him at the grill or in front of his smoker getting ready for some lip-smacking ribs to enjoy with his wife and two kids.

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