A simple drop-in replacement for WordPress' wp_mail function.
To implement this drop-in, add it to your mu-plugins directory. In order for it to work, you must define your Mandrill API key. The easiest way to do this is to add it to your wp-config.php file like so:
define( 'MANDRILL_API_KEY', 'your-api-key' );
This plugin can be installed and managed using Composer; however, because of the way Composer and mu-plugins work, you'll need a bit of a workaround to make sure the plugin is loaded. For more information about how to use this drop-in with Composer, read this blog post by Richard Tape.