eximspamy

EximSpamy is a spam filter framework that utilizes Exim's Perl extension. Notably, this solution requires minimal maintenance and uses very little CPU power. I've been running it on a Pentium 100 MHz server with no issues whatsoever. Unlike other mail filter systems, EximSpamy processes or rejects the mail in a single pass, negating the need for multiple runs through Exim.

Exim's Perl subroutines are executed from filter files, with users specifying a file that contains Perl subroutines in the Exim.conf file. This requires a version of Exim compiled with Perl support enabled, which can be done by adding the line "EXIM_PERL=perl.o" in Local/Makefile of the Exim source tree. Once set up, the system filter (usually system_filter.exim) can utilize the Perl subroutines to execute various tests on any header line (e.g., $header_subject: $header_from:) or message body ($message_body).

EximSpamy utilizes a unique method of catching spam: setting up one user to which all spam-traps are redirected. These are old accounts that are no longer used, or special seeds set up on webpages to catch only spam (such as using white font on white background). All mail sent to the spamtrap is run through a filter that takes a checksum on the subject line, which is then stored in a file (/usr/local/spamy/tmp/sums.txt). Any incoming mail is checked against the known "spam" checksums in /usr/local/spamy/tmp/sums.txt. In other words, the subject line's checksum is computed, and the mail is flagged as spam if the checksum is already known.