There is a way to replicate “email to Evernote” (see here) but it would require you to use some email client that has rule-processing capability, like Apple Mail. You could conceivably sign up for another gmail address just to do this through Apple Mail.
Or you could just fire up Apple Mail for your emails just to use the rule processing, while reading, replying etc through MailPlane.
One simpler method could be to just print that email in MailPlane as hinted by @rmschne and select “Save to Devonthink 3” from the PDF button on the bottom left of the print dialogue box.