add_action( 'phpmailer_init', 'my_phpmailer_example' );
function my_phpmailer_example( $phpmailer ) {
    $phpmailer->isSMTP();
    $phpmailer->Host = 'smtp.example.com';
    $phpmailer->SMTPAuth = true; // Force it to use Username and Password to authenticate
    $phpmailer->Port = 25;
    $phpmailer->Username = 'yourusername';
    $phpmailer->Password = 'yourpassword';

    // Additional settings…
    //$phpmailer->SMTPSecure = "tls"; // Choose SSL or TLS, if necessary for your server
    //$phpmailer->From = "you@yourdomail.com";
    //$phpmailer->FromName = "Your Name";
}