I could not activate the Mail plug-in from DT. I’ve read some source, also in this forum. My problem is, that DT says plug-in is installed. But I have no menu entry in Mail>General>Manage plug-ins. I’ve found no way to enable plug-ins at all. Also I noticed, the mail bundle is not in Library/Application-Support/Mail/… it is now under Library/Mail/Bundle… in opposite what many sources tell for Catalina and also BigSur.
Was there a change in BigSur 11.1? Or is there another way to activate plugins?
Please try to install the plug-in again after first activating full disk access for DEVONthink (see System Preferences > Security). In addition, which version of DEVONthink do you use?