如何使用PHP通过ZOHO api发送电子邮件？

I've followed this doc and here is my code:

$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;

And the response is:

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

And the response means: (according to this)

BAD REQUEST - The input passed in the Request API is invalid or incorrect. The requestor has to change the input parameters and send the Request again.

Any idea how can I fix it?

In order to send mail with Zoho through its API, you need to first authenticate, as seen on the APIDocs:

Note: You can use the API here to retrieve the accountid for the currently authenitcated user.

That said, and refering to your comment, you don't need an SMTP server installed on your server to be able to send mail with PHPMailer:

Integrated SMTP support - send without a local mail server

Source

Zoho requires you to use TLS and the 587 port, so you can set up your connection like this:

<?php
use PHPMailer\PHPMailer\PHPMailer;

$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();