Sending mail through a plugin using wp_mail

Hey guys,

I run several websites on one server and for some reason I cant seem to use wp_mail to send emails through my main domain. Every other domain and new domains I create work but not the main domain. Ive tried using several plugins including wp-mail-smtp but unfortunately I cant get my main domain to send mail using the wp_mail function. I can use the mail() function but this isnt ideal and I heard it can be easily hacked. I wrote a script to test the wp_mail function:

<?php
/**
 * This file can be used to validate that the WordPress wp_mail() function is working.
 * To use, change the email address in $to below, save, and upload to your WP root.
 * Then browse to the file in your browser.
 */

// Set $to as the email you want to send the test to
$to = "support@XXXXX.com.au";

// No need to make changes below this line

// Email subject and body text
$subject = 'wp_mail function test';
$message = 'This is a test of the wp_mail function: wp_mail is working';
$headers = 'no-reply@xxxx.com.au';

// Load WP components, no themes
define('WP_USE_THEMES', false);
require('wp-load.php');

// send test message using wp_mail function
$sent_message = wp_mail( $to, $subject, $message, $headers );

//display message based on the result.
if ( $sent_message ) {
    // the message was sent...
    echo 'The test message was sent. Check your email inbox.';
} else {
    // the message was not sent...
    echo 'The message was not sent!';
}

This function seems to work with my sub-domains but not with my main domain. The error message just says message not sent. Is there a way I can test wp_mail better? Maybe get a more meaningful error message that can help me pin-point the problem code? My hosting provider insist its a wordpress issue.

Kind Regards

UPDATE:
So i’ve reinstalled my core wordpress files and the problem is still there.