If you want to use a 1&1 IONOS domain with mailboxes from a different provider, you simply have to enter the corresponding mail servers in the domain's DNS settings.
Using Different Mail Servers
To use different mail servers, you have to enter the provider's mail servers in the domain's MX record. This record determines which mail servers are responsible for receiving emails for this domain. Consult with your email provider to find out which mail servers you need to enter for your domain.
Entering MX Records
Select a domain from the My Domains page and then click the DNS tab.
Click Add Record.
Select the MX item from the Type list and enter the Host Name, Value,and Priority into the text boxes.
Select a TTL from the drop-down box and then click Save.
Your changes take effect immediately at 1&1 IONOS and should be updated everywhere within an hour.