Use Mandrill SMTP for WordPress wp_mail() function

functions.smtp.php

<?php 
/* Add this to your functions.php or plugin file  */
add_action( 'phpmailer_init', 'configure_mandrill_smtp' );
function configure_mandrill_smtp( PHPMailer $phpmailer ){
    $phpmailer->isSMTP(); //switch to smtp
    $phpmailer->Host = 'smtp.mandrillapp.com';
    $phpmailer->SMTPAuth = true;
    $phpmailer->Port = 587;
    $phpmailer->Username = 'MAILCHIMP_USER';
    $phpmailer->Password = 'MAILCHIMP_API_KEY';
    $phpmailer->SMTPSecure = 'tls';
}
?>