Alright, I had read it, and I had gone and deleted the plugin manually by going on the command line and removing the directory /Users/username/Library/Mail/Bundles/DEVONthink_BigSur.mailbundle.
To no avail, it wasn’t working and I also got the message about a 4.03 update being available for a 4.04 installed plugin.
What did work however, was the trick to go in Apple Mail 15, under ‘Preferences, General, Manage Plug-ins’ and right clicking on the plugin there and then, inside Apple Mail, to ‘Show in Finder’, then deleting it. Then quitting Mail 15. Then re-opening DT 3.8, and re-installing the plugin (this time, DT did not show the error about 4.03 vs 4.04). Then applying the plugin in Mail 15.0, quitting mail, and re-starting it.
I had however previously ran the command in the CLI:
defaults write “/Library/Preferences/com.apple.mail” EnableBundles 1
I do not know if its necessary.
But it works now.
F.