Add an MX record in the IONOS Domain Center so that your emails will be forwarded to Microsoft 365. The address and the corresponding values you need are displayed in the Microsoft 365 Admin Center.
If you have already set an MX Record for this domain and you change the record, new incoming emails will be redirected to your new email addresses. In addition, no emails will be delivered to the email addresses you were using previously. All emails that you have already received will remain saved with your current email provider.