Aha… something I just discovered myself that may help others who find their way to this thread: Mixing non-separator/punctuation ascii symbols into a line that otherwise acts as a separator causes it to pass Discourse’s filtering. So changing a line like:
Header
--------------------------------
To:
--- Header ---------------------
is one way to preserve some form of separator line without having Discourse strip it out. Of course, this still doesn’t help for users who are accustomed to relying on little ---- separator lines to set something apart in an email, so I’m still curious whether others have recommended best practices here.