I’ve posted this elsewhere, but it seems useful here, so at the risk of redundancy:
I use a really easy method to send an email into DT, but it may not be suitable for everyone:
Select the email, Command P (Print), look down at the bottom left, drop down the menu that says “PDF” (which would send it right to the printer), and click on “Save PDF to DEVONthink 3”. A DT3 menu opens up showing Inboxes for every open database, plus all folders in all open databases. File it where you want it. Done.
Obviously this only works if you want the email as a pdf; it isn’t suitable for those who want to reopen the email in Mail.
For that, it’s a relatively easy two-stop process. Drag the Mail onto your desktop, then drag it into the Sorter.
Note: Bluefrog and cgrunenberg. It would be nice if we could drag the mail directly into the Sorter, but it doesn’t work. Don’t know if that’s an Apple problem, or yours.