Thanks for your reply.
Unfortunately, the value of the List-ID header is not a reliable way
to determine that an e-mail originates from a Discourse-powered forum:
-
The CommonMark forums (http://talk.commonmark.org) have List-ID:
<extensions.talk.commonmark.org> -
The Swift forums (https://forums.swift.org) don’t have a List-ID
header (not in their “activity summary” e-mails, at least)
I can’t even do search for occurrences of the “discourse” string in the
headers (which wouldn’t be pretty), since it may not appear at all, as
is the case for the CommonMark forums.