$url = "https://mail.zoho.com/api/accounts/662704xxx/messages";
$param = [  "fromAddress"=> "myemail@mydomain.com",
            "toAddress"=> "somewhere@gmail.com",
            "ccAddress"=> "",
            "bccAddress"=> "",
            "subject"=> "Email - Always and Forever",
            "content"=> "Email can never be dead ..."];
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($param));
$result = curl_exec($ch);
curl_close($ch);
print_r($result);
die;

{"data":{"errorCode":"INVALID_TICKET","moreInfo":"Invalid ticket"},"status":{"code":400,"description":"Invalid Input"}}

<?php
use PHPMailerPHPMailerPHPMailer;

$phpMailer = new PHPMailer(true);
$phpMailer->isSMTP();
$phpMailer->Host = "smtp.zoho.com";
$phpMailer->SMTPAuth = true;
$phpMailer->Username = "your-user";
$phpMailer->Password = "your-password";
$phpMailer->SMTPSecure = "tls";
$phpMailer->Port = 587;
$phpMailer->isHTML(true);
$phpMailer->CharSet = "UTF-8";
$phpMailer->setFrom("mail-user", "mail-name");

$phpMailer->addAddress("mail-to");
$phpMailer->Subject = "subject";
$phpMailer->Body = "mail-body";
$phpMailer->send();