The lastest update for the fast Mac email application Airmail adds DEVONthink integration. Selected email messages can be sent to DEVONthink’s inbox. They are saves as plain text documents and include a link back to the original message in Airmail. Read more about the update here.
On iOS this is also now possible using the new URL schemes that DEVONthink To Go 2.1.3 introduced. Our forum user kseggleton created a custom URL action for Airmail on iOS that sends email as HTML to DEVONthink To Go including email and back links. Read here how to do it.