No Such User Here error when sending via form - php

Question

I have an online form that is being sent to my client's email. Unfortunately I am getting a "No Such User Here" error when submitted. If I test out the form on my email or my coworker's email, it works fine.
The email exists because they have told me they use it to send and receive other email.
Error looks like this:
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
<client's email>
No Such User Here
Edit: For what it's worth, here's my code. I'm using Processwire and using the Form Template Processor module. Which I've used before. This form works perfectly when sent to any email but my client's. I'm thinking this is more of an MX records thing which I'm not super well-versed in.
<?php
$form = $modules->get('FormTemplateProcessor');
$form->template = $templates->get('form'); // required
$form->requiredFields = array('first_name','last_name','company','email','phone');
$form->email = 'CLIENTS EMAIL'; // optional, sends form as email
$form->parent = $page; // optional, saves form as page
echo '<div class="form-header"><h2>Become a member today</h2></div>';
echo $form->render(); // draw form or process submitted form
?>

Answer 1

There are several reasons why you would get a "550 No Such User Here" error.
Mail file permissions are not readable or are incorrect.
The email address was typed incorrectly.
MX records are incorrect ( pointing to the wrong server ).
Email does not route correctly ( Remote / Local domains ).
To dig further kindly go through this article Source
Credits to the Source Poster #James Richardson