The macOS Mail app is an app that supports plugins, and quite a few useful ones have been developed for it. These plugins install in a lot of different ways. Those that are distributed as packages are easy to install; you only have to run the installer and it will be installed. Some plugins are distributed as apps and they’re also simple to install. If you have a Mail plugin that is a .mailbundle folder, it has to be installed manually. Here’s how you can install a Mail plugin on macOS.
Install Mail plugin
Download the plugin and extract it, or mount it and open the mounted volume. Examine the contents of the volume or the folder, whichever it is you have. If the contents have a folder that ends with the .mailbundle extension, you need to move it to the Mail app’s folder.
Open Finder and go to the Library folder for your user. Inside, you will find a folder called Mail. Open it. Inside, there should be another folder called Bundles. It is possible that this folder does not exist. In that case, you can create it yourself. Right-click inside the Mail folder and select New Folder from the context menu. Name it Bundles, and open it.
Cut and paste the .mailbundle folder into the Bundle folder. Log out, and log back into your Mac.
Once you’re logged back into your system, you need to enable the plugin within the Mail app. Open Mail and go to its preferences. On the General tab, click the ‘Manage Plug-ins’ button.
A panel will appear listing all the plugins that are available. Click the checkbox next to the plugin you want to enable and click the ‘Apply and Restart Mail’ button.
Plugins differ from one to another. Some work invisibly e.g., a plugin that sorts your mail, some have an interface of their own. There’s no definitive list of methods for accessing a plugin. After you’ve followed the above steps to install a Mail Plugin, you should consult the documentation for it to see how it works and what changes to the interface, if any, you should expect.
Plugins, like apps, also have compatibility limitations. Make sure the plugin you’re planning to use supports the version of macOS that you’re using. In some cases, if your version of macOS is a bit old, you might have to perform a few additional steps (specific to the plugin) to get it going.
Need to install a plugin for QuickLook? It’s almost a similar process.