Stefan Bauer via Mailman-Users writes:
More and more mails contain s/mime signatures. How to deal with that?
To add to what Mark says, I would say not at all. I would consider broken S/MIME signatures to be a bug, because Mailman should be treating the multipart/signed *part* as a block, and appending both header (if any) and footer (if any) as separate parts. The content subpart should be unchanged so the signature should not be broken. If subscribers complain that signatures don't verify, please report that to us. (I'm not sure we would do anything about it, since Mailman 2 is nearing end-of-life. On the other hand I would consider it a priority for Mailman 3, and it might not be hard to backport.)
The problems with signatures have to do with signatures intended to ensure integrity of the whole message, usually DKIM. These signatures are necessarily broken if Mailman makes any change to the message body or to the specified message header fields. S/MIME only deals with body parts on a part by part basis, and Mailman doesn't need to change those.
Steve