I’m interested in seeing progress around this issue. I hope it will help if I summarise the three interconnected issues that I see here:
In notifications over email:
(1) Distinguish in the email’s subject between PMs with only one recipient and PMs with multiple recipients.
(2) Clarify all the recipients in the email.
The Basecamp solution is to always mention all recipients at the bottom of the email, beside with the option to reply via the web UI.
I really like the latest iteration of the Basecamp wording:
You can reply to this email or __respond in Basecamp__. This message was sent to Alex Armstrong, ABC, XYZ, JKL. __Unsubscribe__ • __Change your notification settings__
I’m not entirely convinced that the bottom part of the message is the best option. It works for Basecamp, but that has fewer types of notifications than Discourse – which will notify you for watched posts, @mentions, @group-mentions, PMs and group PMs, and probably others I’m forgetting. All these should, ideally, be distinguished both in the subject and in the email itself.
In the web UI:
(3) The recipients should be identified near the reply button, at the bottom of the page.
Basecamp handles this with a label that reads: “When someone comments on this message, a notification will be sent to: Alex Armstrong, ABC, XYZ, JKL.”
I’m not sure what a Discourse-ish design might look like, but Cobb has the right idea.