I have successfully tried the process of downloading my email archive to DTPO and it worked like a champ. What I seem to be missing is how to keep the process updated. IOW, I hope there is a process by which the new DTPO email database is updated on a daily basis, adding all that days new emails. Further, I hope this can be made an automatic process.
It’s not automated but you can use the Archive Mailbox function on subsequent days to import the new emails.
You may also want to look at the Mail Rules, though we cannot guarantee they will trigger consistently. (That would be an Apple issue.)
I seem to remember the “archive mailbox” button on the initial setup, but now I can’t remember where it’s located???
DEVONthink Pro Office’s File > Import > Email. It’s a button in the Toolbar.