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I provide SSH shells to a lot of people and block port 25, 465 and so on by default via iptables to avoid that some people send spam messages.

Is there some kind of SMTP filtering proxy or -gateway which limits the rate of messages, send by the individual user and checks the messages against a blacklists based on message header and -content?

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Have you looked at SpamAssassin?

Features

  • Wide-spectrum: SpamAssassin uses a wide variety of local and network tests to identify spam signatures. This makes it harder for spammers to identify one aspect which they can craft their messages to work around.
  • Free software: it is distributed under the same terms and conditions as other popular open-source software packages such as the Apache web server.
  • Easy to extend: Anti-spam tests and configuration are stored in plain text, making it easy to configure and add new rules.
  • Flexible: SpamAssassin encapsulates its logic in a well-designed, abstract API so it can be integrated anywhere in the email stream. The Mail::SpamAssassin classes can be used on a wide variety of email systems including procmail, sendmail, Postfix, qmail, and many others.
  • Easy Configuration: SpamAssassin requires very little configuration; you do not need to continually update it with details of your mail accounts, mailing list memberships, etc. nce classified, site and user-specific policies can then be applied against spam. Policies can be applied on both mail servers and later using the user's own mail user-agent application.
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I will evaluate SpamAssassin, thanks for the tip. –  Robert Marquardt Feb 3 '12 at 22:15

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