Because the digest runner is creating a MIMEMultipart instance on line 175 of src/mailman/runners/digest.py. Should we change that instead?

Good catch. Yes, I think we should change that. What I think I'd like to see is a test that creates a mime digest, and uses that to trigger the attribute errors, rather than creating a MIMEMultipart inside the tests themselves. Does that makes sense? Then the fix would be to change MIMEDigester._make_message() to create the proper subclass. The question then is what that method should return. My first thought is a class that inherits both from MIMEMultipart and mailman.email.message.Message. What do you think? -- https://code.launchpad.net/~abompard/mailman/bug-1130696/+merge/243226 Your team Mailman Coders is requested to review the proposed merge of lp:~abompard/mailman/bug-1130696 into lp:mailman.