Such errors occurs when sending email is rejected due to the username unavailability or not found on the system and also there are other common reasons for getting such a "550 No Such Address Here".
- Incorrect or wrong permissions to Mailbox
- Sender email address was typed incorrectly.
- Mail MX records are not properly configured for the domain.
- Wrong routing of Emails.
I suggest you to contact your hosting providers to set correct MX records for the mail domain or hire a server administrator to setup correctly.