The MX (Mail Exchanger Record) indicates the server through which emails sent to your domain addresses are delivered. If there are several such servers, you can assign preferences (priorities in technical language). A priority is indicated by a number. This means that a server with a lower number has a higher priority.
Email from different domains can work through the servers of different service providers, each service provider has its own MX records for use by its clients. When you send an email, it is sent to the recipient’s email service provider server according to the MX record of the recipient’s domain.
MX record for Zone hosting users – zonemx.eu and priority – 10.