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 is unable to be resolved properly because:

  • the certain DNS records (A, MX, PTR) are missing for the domain
  • the Name Servers of for sending domain are not responding

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

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

The checks in the above-mentioned websites will allow locating the issue with the exact DNS record.

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