$apikey = "Su API";
$sender_name = urlencode("Su nombre");
$sender_email = "tucorreo@dominio.com";
$mail_subject = urlencode("PRUEBA DE API OK");
$mail_body = urlencode("¡HOLA MUNDO! PRUEBA DE CORREO ELECTRÓNICO");

$recipient_email = "correoDestino@dominio.com";

$URL = "https://api.emailroot.com/smtp/mail/send/?apikey=".$apikey."&sender_name=".$sender_name."&sender_email=".$sender_email."&recipient_email=".$recipient_email."&charset= ".$charset."&mail_subject=".$mail_subject."&mail_body=".$mail_body."&format=html";

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $URL);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

eco 1TP4Allput = curl_exec($ch);
curl_close($ch);