I could have quoted a number of posts in this topic for this, but my impression so far of Discourse is that when migrating mailman lists to discourse, each mailman list with its own already email-connected community and established list owners/moderators should get its own discourse install, not just one category in a big discourse install with lots of categories. That way you can let them have full freedom within that community to create whatever categories to organize their discussions and to manage their community rather than forcing them to be part of something bigger and see things from other migrated lists that they potentially have nothing to do with.
The architecture of discourse doesn’t really support the traditional setup where one organization hosts dozens or hundreds of mailman lists, and I’m not sure it’s going to ever make those people happy who just want to live in their email. We can tinker around the edges to make the subject line more customizable, but the tension between what forums need vs what lists need will always be there and I for one would hope that the forum will always win.
This is a bit optimistic - there are other big differences that people who favor email will not be happy with, like the fact that quoted text and forwarded emails get filtered out and don’t get distributed/make it to the forum. You can’t manage your subscription by email, query mailman servers by email to get info, you can’t deduce the email address to reach the moderator etc.
Is it not already in the body of the message, right next to the post title?