There could be a number of reasons why you are getting the '550 No Such User Here' response from the mail server. Here are a few tips for fixing the error:
Email Routing
If your recipient is using a third party email service like Google Workplace, check your email routing settings in your cPanel account.
- If you are routing your your emails to another server, i.e. Google's email server, then try changing your routing settings to Remote Mail Exchanger
MX Record Setup
The issue may be with your recipients MX Record setup.
- You can check your MX records using cPanel
- Ensure this guide from Google is followed
https://support.google.com/a/answer/140034?hl=en
