Murray S. Kucherawy writes:

Essentially to be "DKIM-friendly" you're free to make any changes you want to the message so long as they are confined to those parts of the message not "covered" by the DKIM signature. So if a signature doesn't cover Subject:, you're fine. Obviously there are many MUAs out there that filter/sort based on the Subject: tags that will be affected to some degree, but they could also take the same action on a List-ID: field just as easily.

That's not true for Mailman topics, though. I guess we can add an X-Mailman-Topic field to handle that.