thanks @cgrunenberg
so for the time being: how is one supposed to proceed in order to get things going?
Removing this copy plus the installed plug-in in ~/Library/Mail/Bundles and then reinstalling it should be sufficient.
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This was indeed the solution in my case. 
Did you check the permissions in the Finder?
Are you using a Standard or Admin macOS user account?
Could you guys @cgrunenberg & @BLUEFROG kindly provide an instruction of how to get the DEVONthink_BigSur.Mailbundle eventually working in macOS 11.3.1.
Yes, @BLUEFROG , I am using an Admin Account.
I looked into /Users//Library/Application Support/Mail/Plug-ins/Bundles/Users//Library/Mail/Bundles
and indeed found:
1 = Name of mailbundle
2 = Sharing & Permissions: “You have unknown access” 
What am I to do about this? @wernervp, @BLUEFROG, @cgrunenberg
I deleted this folder. You wil probably be asked for your password. After this try reinstalling the plugin, it worked for me.
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as I tried to illustrate with the image above: “You have unknown access” = I CAN’T DELETE IT @wernervp
You could try to do this from the Terminal. (Please be aware using ‘rm’, especially as root. Only use it when you know what you are doing.
- Open the terminal, you can find this in: Applications → Utilities
- sudo rm -rf /Users//Library/Application Support/Mail/Plug-ins/Bundles/Users//Library/Mail/Bundles/DEVONthink_BigSur.mailbundle
I would suggest you are more cautious in suggesting a course of action like this. rm in UNIX is not to be trifled with and one misstep could be disastrous.
Users//Library… ?
This isn’t a valid filesystem path. 
Also, the unknown access would be an Apple issue, not something caused by DEVONthink.
The correct path contains the user name between the // (so twice within the path as posted). rm probably is the solution, but I join you in urging caution when suggesting to somebody who might or might not know what sudo rm -rf is doing to use it without further warning 
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Indeed, the username was filtered because I used < username > (without the spaces) with was filtered. The path indeed is: /Users/your-user-name/Library/Application Support/Mail/Plug-ins/Bundles/Users/your-user-name/Library/Mail/Bundles.
Yes, I agree… using rm needs a warning before usage
Thanks for the clarification.
ok – here’s the solution
- rename the last “Bundles” to “Bundles_”
- reinstall the Apple Mail Plugin
… and you’re got to go.
Still does not work for me in BigSur 11.6, DT 3.8, Mail 14.0
Tried this full cleanup without help:
- Confirm Full Disk Access and Automation enabled for DT in System Settings (also tried removal and addition - no help)
- Quit DT, quit Mail
- Rename BOTH Bundles to Bundles_
PATH1: /Users/YOUR_USER/Library/Mail/Bundles
PATH2: /Users/YOUR_USER/Library/Application Support/Mail/Plug-ins/Bundles/Users/YOUR_USER/Library/Mail/Bundles - Run Mail and verify that there is NO DevonThink plugin in Settings / General / Manage plugins
- Quit Mail
- Run DT and verify that in DevonThink 3 / Install Add-Ons checkbox at “Apple Mail Plugin” is enabled and available (thus ready to be installed)
- Confirm installation with “Install” button
- Confirm that Bundles showed up in PATH1
- Run Mail and enable DT_BigSur plugin in Settings / General / Manage plugins, and of course allow Mail to restart itself
- Confirm that Bundles showed up in PATH2
- Try to drag & drop an e-mail into DT
→ NOPE! - Open any e-mail and choose “Mailbox / Add to DevonThink 3”
→ “Archive emails” blinked for a fraction of a second in DT on bottom left corner and system message showed up from Apple Mail that “Imported 0 messages”
one comment - I find out that the e-mail I tried does not work, but many others I can drag & drop into DT.
This e-mail is a plain text so it is very strange. “Mailbox / Add to DevonThink 3” does not work at all.
Select an email in Apple Mail.
Choose Add message(s) to DEVONthink 3 in the global script menu in the menubar at the top of your screen.
Are you prompted to allow Mail to control DEVONthink?
If so, allow it.
Does that help?
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