[Mailman-Users] List security

Hi,

I'm running a list on Mailman 2.1.3 and I am looking to secure my list as much as possible. I saw this in FAQ 3.11 (http://www.python.org/cgi-bin/faqw-mm.py?req=show&file=faq03.011.htp):

A more secure alternative is for your approved posters to add an Approved header to their postings as a header, or as the first line of the post).

I'd like to implement this but I'm uncertain to go about it. I'm pretty certain the content filtering options are where the test for the Approved header should go. My question is how can the header be added to the posting? Any insight or tips as to how this can be accomplished will be appreciated.

Thanks, Sean