What do I need to do before/after upgrading to macOS Monterey?
There is not much you need to do before upgrading to macOS Monterey. Please make sure that you are using the latest Mailbutler version by checking for Updates in your Mailbutler preferences.
To check if you are on the latest version, open the Mailbutler sidebar, click on the Account Settings (⚙) button, then Check for Updates.
After upgrading to macOS Monterey, you will only get a notification from Mailbutler to reactivate the plugin in your Apple Mail preferences.