Note: Please note that this is not a Mailbutler issue, and we cannot offer support for it. If you experience the issue and the solution in the article doesn’t help, please contact Apple Support.
To be able to use Mailbutler in Apple Mail, on macOS Mojave (10.14) and later, you first need to enable Mailbutler. Here is an article about how to activate plug-ins in Apple Mail.
However, some users have reported that they can’t enable Mailbutler.mailbundle in Apple Mail because the Manage Plug-ins button is missing from Mail’s General Preferences view.
The issue is caused by macOS, and can happen even if you’re not using Mailbutler at all. To solve this issue:
1. Quit Mail.
2. In the Terminal, type in:
sudo defaults write “/Library/Preferences/com.apple.mail” EnableBundles 1
defaults write com.apple.mail EnableBundles -bool true
defaults write com.apple.mail BundleCompatibilityVersion 4
3. Open Mail, then go to Preferences->General. The Manage Plug-ins button should now be there.