How do I resolve the 550 or 553 Relay Denied error?
This article will explain how to resolve one of the most common SMTP email server errors
If you are getting the error message ‘550 Relay Denied’ or ‘553 Relay Denied’, it means that the outgoing mail server cannot verify who you are and will not allow you to send mail.
To fix this, you should use the SMTP server for the provider you have used. For 123 Reg, this will be smtpout.secureserver.net (Generation 2 mailboxes only) or smtp.123-reg.co.uk (Generation 1 mailboxes only).
Most Internet Service Providers restrict access to their outgoing mail servers to prevent spam from being sent through their mail servers.