Comment 1 for bug 1444673

There is a work around for Mailman 2.1.17 and newer (older versions don't have ANONYMOUS_LIST_KEEP_HEADERS). Copy ANONYMOUS_LIST_KEEP_HEADERS from Defaults.py to mm_cfg.py if it's not already there. You will then have something like the following (possibly edited by you already).

ANONYMOUS_LIST_KEEP_HEADERS = ['^(?!x-)', '^x-mailman-',
                               '^x-content-filtered-by:', '^x-topics:',
                               '^x-ack:', '^x-beenthere:',
                               '^x-list-administrivia:', '^x-spam-',
                              ]

In this list, replace the first line with

ANONYMOUS_LIST_KEEP_HEADERS = [
                       '^((?!x-)(?!dkim)(?!authentication)(?!domainkey))',
                       '^x-mailman-',

and keep the next 4 lines as is. I.e., you are just replacing '^(?!x-)' with '^((?!x-)(?!dkim)(?!authentication)(?!domainkey))'