Perhaps we should not be stripping attachments but encapsulating the whole message with enough DKIM signed meta data to enable DKIM processing to work a the far end after DKIM verifying the mailing list input first. This gives you a trust chain.
Those are separate issues. We strip attachments because people send messages that say "my kid's school play is tomorrow at 7 PM, tickets $5, and here's a poster" and attach a 20MB powerpoint horror.
Wrapping messages to make them a one-message digest is 100% standards compliant, and is one of the options in Mailman. The only problem is that most MUAs render them unspeakably badly.
This is very well trodden ground. Look at the Mailman developers' list where this has all been discussed to death.
Regards, John Levine, johnl@xxxxxxxxx, Taughannock Networks, Trumansburg NY Please consider the environment before reading this e-mail.