How to Install a Mail Plugin on macOS

If you want to install a Mail plugin on a Mac today, the supported route is usually to install the Mac app that contains the extension, then enable that extension inside Apple Mail. The older method of copying a .mailbundle folder into your user Library belongs to the legacy plug-in system and should not be treated as the default on current macOS versions.

Apple’s current Mail documentation calls these add-ons Mail extensions. They can add compose tools, message actions, content blocking, or message-security features. Apple documents managing them from Mail > Settings > Extensions. See Apple’s Mail extensions guide and the MailKit documentation.

How to install a Mail plugin on macOS

1. Install the app that contains the Mail extension

Start with the developer’s current download. Many Mail extensions are bundled inside a normal Mac app, so you install the app first. If it comes from the App Store, use the normal Get or Install button. If it comes directly from the developer, open the downloaded .dmg or .pkg and follow the installer’s instructions.

Do not download an old copy of a plug-in from an unofficial mirror just because it includes a .mailbundle folder. For software downloaded outside the App Store, macOS uses Gatekeeper and notarization checks to help verify the developer and detect known malicious software. Apple explains these protections in its guide to safely opening apps on a Mac.

2. Open Mail Extensions settings

Open Mail, then choose Mail > Settings from the menu bar. Select Extensions. Apple lists available Mail extensions here, along with information about the selected extension.

If you do not see an Extensions pane, first confirm that the add-on actually supports your macOS version and that you installed the latest release from its developer. A legacy plug-in may not expose a modern Mail extension at all.

3. Enable the extension

Select the extension and turn it on by checking its box. Apple states that enabling or disabling a Mail extension takes effect immediately, so the old instruction to click “Apply and Restart Mail” is no longer the general workflow for current Mail extensions.

If the extension needs account access, message permissions, or an initial setup step, follow the developer’s own prompts. Only grant permissions that make sense for the feature you are installing.

4. Use the extension in Mail

Where the extension appears depends on what it does. A compose extension may add a control to a new-message window, while other extensions can act on incoming messages, block remote content, or provide signing and encryption features. Apple notes that some extension buttons can be added by customizing the Mail toolbar if they are not already visible.

What if the download is a .mailbundle folder?

A .mailbundle is associated with Apple Mail’s older plug-in architecture. Older instructions commonly told users to create a Bundles folder under ~/Library/Mail/, copy the plug-in there, sign out or restart Mail, and then use a Manage Plug-ins button. That process is not a good general recommendation for a current Mac.

If your add-on is available only as a .mailbundle, check the developer’s documentation for explicit support for your exact macOS release. If the developer offers a newer MailKit-based version, use that instead. Avoid manually moving plug-in folders into hidden Library locations unless the developer specifically requires it for the OS version you are running.

If the Mail extension does not appear

• Update the extension app: install the newest release that supports your version of macOS.
• Open the companion app once: some products finish setup only after their main app has been launched.
• Check Mail > Settings > Extensions: make sure the extension is listed and enabled.
• Check the toolbar: if the extension should add a button, customize the Mail toolbar and look for it there.
• Avoid forcing incompatible legacy bundles: an old .mailbundle may simply not be compatible with a modern release of Mail.

How to remove a Mail extension

Go to Mail > Settings > Extensions, select the extension, and use the uninstall option when available. Apple also notes that deleting the app that contains the extension removes that extension. If you only want to stop using it temporarily, clear its checkbox instead.

The key difference from older guides is simple: for current macOS, install the developer’s supported app, then manage its Mail extension from Mail settings. Treat manual .mailbundle installation as a legacy, version-specific exception rather than the standard method.