MX (an acronym for Mail Exchanger) records are used for email. They specify which mail servers send and receive your emails. If you would like to use MX records other than the IONOS default records, you can do so in the IONOS Domain Center under DNS Settings.

Default Settings:

By default, the IONOS mail servers are set for email addresses created with your IONOS domain, so you do not have to take any extra steps if you want to use them.