Thanks for sharing the instructions for Mojave
I have one question: does Devonthink3 make an actual copy of the email in its database or just creates a link to the actual file ?
“Apple Mail Plugin: A plugin for Apple Mail that allows you to archive messages or whole mailboxes directly from within Mail. On Mac OS Mojave you’ll have to manually activate the plugin after installation. In Apple Mail visit Preferences > General > Manage Plug-ins and activate DEVONthink’s plugin.
Excerpt From: Eric Böhnisch-Volkmann. “DEVONthink Documentation”. Apple Books.