It can be anything! An apex, subdomain, or even a URL.

Results forfallback.mail.pcextreme.nl

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Name	TTL	Value
fallback.mail.pcextreme.nl	60	185.66.250.200Your Hosting B.V / Your Hosting B.V.
fallback.mail.pcextreme.nl	60	185.66.250.201Your Hosting B.V / Your Hosting B.V.
fallback.mail.pcextreme.nl	60	185.87.184.60PCextreme B.V.
fallback.mail.pcextreme.nl	60	185.87.185.193PCextreme B.V.
fallback.mail.pcextreme.nl	60	185.87.186.222PCextreme B.V. / Your Hosting B.V.
fallback.mail.pcextreme.nl	60	185.107.224.99PCextreme B.V. / PCEXTREME