When saving one of the following:

Card
Contact person
Sale order
Sale shipment
Purchase order
Email batch

You may see this error message:

This error message indicates that the email address domain is not valid or does not have valid MX records defined (and therefore an email sent to that address would bounce).

You can check the validity of the email domain at https://mxtoolbox.com/MXLookup.aspx. Just type in the domain component (the part after the @ sign) and click "MX Lookup". This will provide a report as to why the domain is failing validation.

Browser not supported