You can install Postie via the plugin store, or through Composer.
To install Postie, navigate to the Plugin Store section of your Craft control panel, search for
Postie, and click the Try button.
You can also add the package to your project using Composer.
composer require verbb/postie
You can try Postie in a development environment for as long as you like. Once your site goes live, you are required to purchase a license for the plugin.
For more information, see Craft's Commercial Plugin Licensing.
Available for Craft 3. Get it from the plugin store.