You can somewhat replicate the Mail-to-Evernote experience on your Mac:
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In the Contacts app set up a new contact (click “Company” and give the “Company name” something like “2Devonthink” or “2DT”).
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In the email address field, let’s say your email address is “johnny.appleseed@icloud.com”, just add “+DT” or “+Devonthink” after the name before the “@”. So in this example it would be “johnny.appleseed+DT@icloud.com”. This is a little known fact that the “+” and anything after it up to the “@” is ignored by mail servers.
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In Mail on your MacOS machine create a rule:
A. If “To” is equal to “johnny.appleseed+DT@icloud.com”;
B. “Run Applescript” “Mail Rule - Add messages to DEVONthink” (this script is part of the DT3 add-ons);
C. “Move message” to trash.
So when you are out and about on your iPhone, just forward the message to the contact you had set up like you would to your mail-to-Evernote address. The message will be processed by Mail if and when it is running and you should get your email imported into the DT3 Inbox.
From there you can create smart rules to file the imported mail automatically. Of course, you could vary the “+your-text-here” part in the name and perhaps use a different one for each subject or database (like an EN Notebook) or tag. Or combine it with text that you add before forwarding to aid filing.