Since macOS Mojave (10.14), it is now required to activate Apple Mail plugins before using them. To start using your inbox with Mailbutler, you’ll need to first enable Mailbutler.mailbundle manually through your Apple Mail Preferences and restart Mail.
Note: This tutorial shows how to activate Mailbutler on Mojave and Catalina, but the steps are the same for every Apple Mail plugin.
Activate Mailbutler plugin on Apple Mail
1. Open Apple Mail.
2. Click Mail in the menu bar on the top left of your screen.
3. Go to Preferences…
4. Click on Manage Plug-ins in the General tab.
5. Activate the plugin Mailbutler.mailbundle by checking the checkbox.
6. Click Apply and Restart Mail.