The following non-delivery report:

450 4.1.8 Sender address rejected: Domain not found.

Sender address rejected

indicates that the email is rejected because the sender's domain cannot be resolved properly because:

  • A and MX records are missing for the domain
  • the Name Servers of the sending domain are not responding

The domain might belong to your account or be an external one.

Note: the increase of these bounce-back messages is possible due to the recent migration from McAfee Email Protection to Email Protection which, unlike McAfee, performs the checks for valid A and MX records.

In order to check if there are any issues with resolving DNS records for the domain, the following websites can be used:

To resolve the issue, the sender needs to provide the results of the A and MX records check to their DNS hosting provider to configure them properly.