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Howtos - Filtering mail - tricks and tips

[edit] Overview

The ClarkConnect system provides a variety of tools to filter e-mail (in addition to the already existing antispam and antivirus tools). This howto will cover two types of filtering:

• Global filtering
• Per-user filtering

[edit] Global Filtering

The Postfix mail server provides built-in mail filtering that is performed as the mail message arrives. Typically, this filter is used to stop worms, viruses and other easy-to-identify messages. More sophisticated filtering (for instance, decoding attachments for analysis) is not performed at this stage of mail processing. Pre-filtering, in other words, has the following characteristics:

• Helps preserve server resources by rejecting mail early in the process
• Provides simple but limited filtering capabilities

To enable the built-in pre-filtering tool, make sure the following line is listed in /etc/postfix/main.cf:

header_checks = regexp:/etc/postfix/header_checks

In our example, we are going to discard mail messages with the following subject line: Possible SPAM (accuracy very high). The header_checks file looks like:

/^Subject: Possible SPAM \(accuracy very high\)/ DISCARD

After making a change to the header_checks file, restart the Postfix mail server:

/sbin/service postfix restart

Messages with the offending subject line will now be silently discarded. To see all the options available with this kind of pre-filtering, please review the header_checks documentation.

[edit] Per User Filtering

The webmail application in ClarkConnect provides tools for creating server-based filtering rules on a per user basis.

[edit] Links

• header_checks