I use a variety of methods to get an email into DT depending on what I want to do:
Select the email. Command P: In dropdown menu at bottom, select Save pdf to DevonThink 3. In the next drop down menu, choose where to put it in a database. Works great as long as you don’t plan to reopen it in Mail.
Use the Mail Save As command, for saving it as an rtf in Finder. Ditto Export as pdf.
Drag either of those into the Sorter if you don’t have an open database. Two steps instead of one, but it works fine.
If you drag an email into Finder (say on your Desktop), you get an .eml file that you can then drag into a database or the Sorter and reopen in Mail.
Did this in a hurry; hope it’s clear and I didn’t get something wrong.