There was a feature that I haven’t used in a couple years: “import email.” What it would do (I think) is connect directly to my account, or look directly in Mail.app, and import all of the emails in there. It would skip any emails that it already had in the database.
Now the only email import option I see is “Unix mailbox” and that’s not the same thing…
Assuming you are using DT3 (Pro or Server, but not Vanilla), it’s here:
Clicking on the arrow opens up the drop down and shows - on my Mac - Apple Mail. if it doesn’t on your Mac, it’s possible you either need to activate Full Disk Access for DT in System Preferences/Security & Privacy/Privacy (or possibly allow Automation of Mail by DT in the Automation section in System Preferences/Security & Privacy/Privacy). It is also possible (I can’t remember) that you need to install the Apple Mail Plugin (in the DEVONthink 3 menu, Install Add-Ons…); it is certainly recommended, I just can’t remember whether it is requisite.
That’s it! Thank you very much 
