I don't think we want to allow external subscriptions by email. To support this functionality (which I want too!) we'll either do something similar to what we did with mail-archive.com (auto-subscribe gmane), or we'll ask mail-archive.com to forward messages to gmane (which i believe is an option we decided not to do at first). Or we'll work on our own NNTP bridge to the archives. This latter is being worked on in upstream Mailman 3 (or perhaps as a separate Pipermail project).
I don't think we want to allow external subscriptions by email. To support this functionality (which I want too!) we'll either do something similar to what we did with mail-archive.com (auto-subscribe gmane), or we'll ask mail-archive.com to forward messages to gmane (which i believe is an option we decided not to do at first). Or we'll work on our own NNTP bridge to the archives. This latter is being worked on in upstream Mailman 3 (or perhaps as a separate Pipermail project).