Exim and Spamassassin: Rewriting headers, adding SPAM and Score to Subject

This tutorial is a follow-up to my article Setting up Exim4 Mail Transfer Agent with Anti-Spam, Greylisting and Anti-Malware.

I finally got around solving this problem: If an email has a certain spam score, above a certain threshold, Exim should rewrite the Subject header to contain the string *** SPAM (x.x points) *** {original subject}

Spamassassin has a configuration option to rewrite a subject header in its configuration file /etc/spamassassin/local.cf  …

rewrite_header Subject \*\*\*SPAM\*\*\*

… but this is misleading, because it is used only when Spamassassin is used stand-alone. If used in combination with a MTA (Mail Transfer Agent) like Exim, the MTA is ultimately responsible for modifying emails. So, the solution lies in the proper configuration of Exim. To modify an already accepted message, the Exim documentation suggests a System Filter. You can set it up like this:

Enable the system filter in your main Exim configuration file. Add to it:

system_filter = /etc/exim4/system.filter
system_filter_user = Debian-exim

Then create the file /etc/exim4/system.filter , set proper ownership and permission, then insert:

if $header_X-Spam-Score matches "^\[^-0\]\[0-9\\.\]+" and ${sg{$header_X-Spam-Score:}{\\\\.}{}} is above 50
then
headers add "Old-Subject: $h_subject"
headers remove "Subject"
headers add "Subject: \*\*\* SPAM ($header_X-Spam_score points) \*\*\* $h_old-subject"
headers remove "Old-Subject"
endif

This means: If the header $header_X-Spam_score_int  is present (has been added by Exim in the acl_check_data  ACL section, see my previous tutorial), and is more than 50 (this is 5.0), rewrite the Subject header. The regular expression checks if the spam score is valid and not negative.

Note that in the acl_check_data section of the Exim config, you can deny a message above a certain spam score threshold. This means, in combination with this System Filter, you can implement the following rules:

• If spam score is above 10, reject/bounce email from the ACL.
• If spam score is above 5, rewrite the Subject.