Messagelabs customers are unable to email another client of the ESS when their domain is still active but pointing to another vendor's MX records.
Error received: "421 4.4.0 [internal] no mail servers for this domain could be reached at this time". This error is common, but mostly will be seen in such a scenario.
Email Security.cloud
The receiving domain is not using Messagelabs clusters, but they are pointing to a different MX records.
There are 2 solutions in order to quickly resolve the Issue :
1 - The recipient's domain owner or Administrator should remove the domain from the client net portal, if they are sure that the domain will not be using our services.
2 - The domain should be using our MX records as assigned during the provisioning process.
Note: If the end customer is not willing to use our services and they would like to cancel it, We recommend to contact Support or raise a Non technical case.