Mac Montery,
the mail plugin library in the library folder is empty and therefor I can not activate it.
Devonthink has access to the drive.
In Devonthink the program says that the plugin is installed (checkbox is grey), but it is not.
I can delete the mail plugin.
I can then add a new mail plugin from DEVONthink 3.
In Mail I can not see in the preferences the Mail Plugin.
The name of the mail bundle is DEVONthink_BigSur.mailbundle
I am using MacOS Monterey.
The name of the mailbundle is inconsequential. It refers to a division of the operating system where the internals differed based on changes made by Apple.
Is your home directory on the internal boot drive or an external drive?
Steps 1 through 3 are fine but the is a difference with step 4
if I go to Library the Mail folder with the DT3 bundle is not there. It is in ~/Library/Mail though and when I delete it and start DT3 it does place it back in that same location.
It does not appear in the apple mail settings extensions tab though.
my home directory is on the internal drive of my M1 MacBook directly under /users
I am running Ventura 13.2 and DT3.8.7
I have reinstalled both and tried the instruction sequence again but still get the same result. Bundle installs in Library/Mail under my home directory not in /Library and apple mail sees no extensions
There will be nothing related to DEVONthink in the Extensions pane as we do not have an extension. @chrillek is correct and your screen catpure shows the plugin is installed and enabled.
You should now file Add to DEVONthink 3 commands in the Mailbox and Messages menus in Apple Mail.