We should mirror the archives of and facilitate participation on external mailing lists.
Should avoid subscribing to the same list twice to save SQL space etc, though. Adding a mirrored list may be something you have to ask an admin to set up for you?
Yes. The lists are explicitly designed to prevent this, because there are a lot of problems with it. lists.sr.ht is designed to be the canonical list, and if it's not it may exhibit some behaviors which are not desirable, such as annoying senders about attachments and bad mimetypes.
There was some WIP work to make lists.sr.ht support mirrors as a first-class feature, but it was never finished. You're welcome to pick this up if you wish.
I have a hint on why this might be broken: the use of unixfrom and smtp.sendmail rather than send_message in blueprints/user.py.