Using Discourse in a mailing list mode generates multiple emails for a single user. This quickly becomes unmanageable and raises the need to identify the Groups or Category Slugs of the messages to quickly jump to the correct message without having to click through all messages. Using Category Name in the subject line is not very useful, since Category names are long, sometimes similar and get truncated when you are looking at the list of messages.
It will be very useful if %{group_name} or Category slug can be added to the email subject when creating a new topic and retained when replying to a post. It would also help if it can be added to the email footer.
(Currently %{group_name} can be added to the subject line if you are sending a Personal message, but that feature is not useful here.)