Please open the macOS Mail App and then open Settings/Preferences. If you need to login to an email account, you can simply click on the Other option and provide some random information to be able to gain access to Mail's preferences. Once on the General Preferences window, at the bottom you will see the option to "Manage Plug-ins", click on it and tick/enable the SideloadlyPlugin.mailbundle and then click on Apply and Restart Mail.