550 5.7.1 Relaying denied

The sending client IP is not authorized for the current SmartRelay configuration. Please get in touch with your usual contact at Contactlab, such as your project manager, account manager or Contactlab customer care, to make sure you have pre-authorized your origin IP addresses. See IP-based access control lists (ACLs) for more details.

This is a permanent error. Re-sending the message will not succeed unless the origin IP is pre-authorized with Contactlab.