Subdomains that start with an underscore cannot be created at 1&1 IONOS.
Some providers, however, require you to configure a CNAME record for a host that begins with an underscore. This article will show you how to configure such a CNAME record. You have to use the 1&1 IONOS name servers, and you do not need to have previously created a subdomain.
Creating a CNAME Record for a Host Name in My 1&1 IONOS
Select a domain from the My Domains page and then click the DNS tab.
Click Add Record.
Select the CNAME item from the Type list and enter the name of the subdomain in the Host Name text box. This does not contain the domain name. Example: If the subdomain is _domainkey.example.com, then the subdomain name is _domainkey.
Then in the Value box enter the host name to be saved as the destination in the CNAME record.
Select a TTL from the drop-down box and then click Save.
The procedure described is only for configuring CNAME records for host names that contain an underscore.
Because of the decentralized structure of the domain name system, it can take up to an hour before the change takes effect everywhere.
The subdomain you created is stored only in the DNS zone file. It cannot be used with 1&1 IONOS products.