This Article describes a scenario where You do not receive an email containing the DNS validation key.
Email Security.cloud
The mail containing the validation key could be blocked by the receiving server.
Few steps to check in order to resolve this Issue :
1 - It is recommended to check the queue at the client's server
2 - Remove any block policy if it exists
3 - Release the mail or resend it again from the Add Domains wizard in ClientNet portal.