Short Explanation: The greylists forces messages sent via a Windows server to validate the message. When the Windows server validates the message, it is sent to it's intended recipient and the sending server is whitelisted by our receiving server.
Longer Explanation: A "Windows server greylist" is a filter on our incoming email server that, when receiving an email from a Windows mail server, will immediately bounce that message back to the sending server and request that it is resent (the actual bounced messages says: "451 please try again later."). This is not an outright rejection and in most cases of a valid message, the sender will not see this bounce back message at all. Their sending mail server will simply follow the request of our mail server and "try again later". When the mail server sends the message again at that later time (usually only a couple of minutes), our server will then accept the message and whitelist that server, thereby allowing messages from that server to be sent unimpeded to our mail server.
Why we are doing this: The large majority of spam is sent from Windows servers that do nothing but send spam email. These servers will not "try again later" because the last thing a spammer is going to do is deal with bounce back messages, server error messages or anything that may distract from the process of sending spam.
These preventative measures are necessary due to the amount of spam being sent on the internet. 96.9% of messages sent to our mail server are unquestionably spam sent from blacklisted servers, while another 2% fall into the questionable range.
The Minus: Since doing this, we have encountered a handful of customers telling us that people sending them mails are receiving these bounce back messages. If the sender is receiving those, then their outgoing mail server isn't setup properly to deal with 451 error messages and they should contact the person in charge of maintaining that server.
The Plus: In large part because of the greylist, we've received many customers letting us know that the amount of spam they receive has dropped as much as 90%.