Creating Subdomains in the Domain Center and Configuring CNAME Records for Microsoft 365
To use an MX record together with the CNAME Records, you need to create a subdomain for the particular CNAME record in the IONOS Control Panel.
You then have to configure the corresponding CNAME record for these subdomains. The names of the required subdomains are listed in the Microsoft 365 Admin Center on the Update DNS Settings page under CNAME Records in the Host column. The table also contains the addresses and values you will need for the CNAME records.
Adding Subdomains in the IONOS Domain Center
You can find more information about setting up a subdomain here.