Using greylisting to reduce spam
Have you ever received an email you didn’t ask for, from a sender you have had no prior contact with, maybe offering a product you do not want? Then you may be the victim of SPAM!
I’m sure you’ve heard of spam, most of us get them by the dozens or hundreds a day. The “good” thing about spam and spam senders is that they are rarely standard compliant. This means something called greylisting works very efficiently at reducing spam.
The theory is that legit mail servers resend on temporary errors, while spam senders do not. So the greylisting software refuse the mail with a temporary error the first time, and accepts it after some time have elapsed. For more information please read the whitepaper by Evan Harris
Sendmail has an system called milter for filtering mail in the mail processing chain. A mail filter can be used for filtering out spam, viruses, etc. A milter can reject a message during the SMTP session. There are some milter implementations of greylisting. On of them is milter-greylist.
If you’re running your own mail server, greylisting is definitely something to consider in your fight against spam.