Force mail to re-initialize the plugin.

Quit Mail

Right click on the Finder icon in the Dock and click “Go to Folder...”

Copy and paste this path: ~/Library/Application Support/Mail/Plug-ins

Rename “Bundles” in the opened folder to “Bundles-DISABLED”

Open Mail and go to Mail Preferences

Click Manage Plugins and enable AltPlugin

Click “Apply and Restart”

Profit!