GPG Mail not in Manage Plug-ins list after installation or doesn't remain active

What you are seeing is a bug which occurs if during the installation of an older GPG Suite SIP was deactivated, but is now activated.

To fix this problem:

  1. Close Mail.app
  2. Open System Preferences › Security › Privacy
  3. Enable Terminal under Full Disk Access
  4. Restart Terminal when prompted
  5. Run the following command:

    sudo rm -rf ~/Library/Containers/com.apple.mail/Data/DataVaults

  6. Open Mail

If you are seeing the error "Operation not permitted", you will have to temporarily disable SIP and repeat step 5.