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You are here: Home About Systems Greylisting

Greylisting

by Ward Vandewege Contributions Published on Feb 06, 2008 04:45 PM

What is greylisting?

Greylisting is a technique to combat spam. Greylisting consists of temporarily refusing e-mail if the combination of sender e-mail address, originating mailserver, and destination e-mail address has not been seen before. Because our mailservers issue a temporary error, all mailservers that respect internet mail standards will try to redeliver the message a bit later. Our servers are configured to allow the message through 20 minutes after it was first temporarily refused.

Why does this help against spam? Most spam originates from Microsoft Windows machines infected with some sort of virus or trojan horse software. That software is not very sofisticated and does not implement a standards-compliant mailserver. It just tries to deliver as much spam as possible, and does not care to queue the spam for a redelivery attempt if it can not be delivered immediately.

For more background on greylisting, see this wikipedia article.

You received a bounce e-mail with this url in it

If you received a bounce e-mail with this url in it, your mailserver is misconfigured. It interpreted the temporary failure as a permanent error and did not retry delivery of your message. Please contact your systems administrator to get this fixed.

When does the FSF apply greylisting?

We have configured our mailservers to apply greylisting to incoming mail if it meets at least one of the following criteria:

  • The machine that is trying to deliver the e-mail to our server runs a version of Microsoft Windows
  • The machine that is trying to deliver the e-mail to our server does not have a reverse dns entry
  • The reverse dns entry of the machine that is trying to deliver the e-mail to our server makes it look like its IP is dynamic
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