This should be possible via File > Import > Unix Mailbox….
After adding this account to e.g. Mail, this shouldn’t be a problem via View > Sidebar > Import > Email but importing from server-based accounts might require quite a while.
DEVONthink doesn’t link to any online services but the local iCloud Drive folder can be indexed.
All possible.
Finally, the trial of DEVONthink is fully functional and runs for 30 days or 150 hours of runtime (whatever lasts longer), therefore it’s probably better to test this on your own.