[Mailman-Users] Adding custom header

[Eric Smith]

I want to receive all mail relating to lists that I create and
manage into my inbox. Because there are arbitrary list names and
also numerous domains, I think that the best way is for all list
mail to carry an identifying mail header.

Having searched the archives, I understand I should 
edit the source.
I am thinking like appending a tag to an existing header like
List-Id:

Is this giving the corre t file to edit.
 $ find /usr/lib/mailman -type f -exec grep -l List-Id {} \;
/usr/lib/mailman/Mailman/Handlers/CookHeaders.py

Any caveats, guidelines, bearing in mind this:
http://mail.python.org/pipermail/mailman-users/2011-December/072595.html

-- 
Eric Smith