If you use DEVONthink on your Mac and have recently updated to Mojave, you may have noticed a disconnect between the application and Mail. This is down to the new security features that have been implemented in Mojave and requires just a little intervention on your part to remedy. It’s quick and easy.
1 - Go to System Preferences -> Security and Privacy and enable full disk access for DEVONthink.
2 - Install the Mail plugin from the Add-ons within DEVONthink
3 - Enable the Mail plugin by opening Mail -> Preferences in the screenshots below