I was reading about Postfix’s cleanup facility which supports header_checks which can be specified in a regexp: table. And it inspired me to come up with this header_filter_map file:

/^Date: .* [JFMASOND][aepuco][nbrynlgptvc] 1/ REJECT
/^Date: .* [JFMASOND][aepuco][nbrynlgptvc] 200/ REJECT
/^Date: .* [JFMASOND][aepuco][nbrynlgptvc] 2010/ REJECT
/^Date: .* [JFMASOND][aepuco][nbrynlgptvc] 2011/ REJECT
/^Date: .* Jan 2012/ REJECT
/^Date: .* Feb 2011/ REJECT
/^Date: .* Mar 2011/ REJECT
/^Date: .* Apr 2011/ REJECT
/^Date: .* May 2011/ REJECT
/^Date: .* Jun 2011/ REJECT
/^Date: .* Jul 2011/ REJECT
/^Date: .* Aug 2011/ REJECT
/^Date: .* Sep 2011/ REJECT
/^Date: .* Oct 2011/ REJECT
/^Date: .* Nov 2011/ REJECT
/^Date: .* Dec 2011/ REJECT

Which I applied in Postfix by adding the following line to /etc/postfix/main.cf:

header_checks = regexp:/etc/postfix/header_filter_map

It remains to be seen if what I’ve done will work, and at the moment this is a bit of a pain because I have to manually update the header_filter_map file every month, but the general idea is that if the regexp matches a date too far in the past then the message is rejected. Hopefully then those spammers who have messages turning up in my history will be gone.