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

Results forsmtp.google.com

Resolver

Location

A

Name	TTL	Value
smtp.google.com	300	2607:f8b0:4004:c06::1aGoogle Public DNS (iad) / Google LLC
smtp.google.com	300	2607:f8b0:4004:c06::1bGoogle Public DNS (iad) / Google LLC
smtp.google.com	300	2607:f8b0:4004:c21::1bGoogle Public DNS (iad) / Google LLC
smtp.google.com	300	2607:f8b0:4004:c23::1bGoogle Public DNS (iad) / Google LLC

Name	TTL	Value
smtp.google.com	3600	google-site-verification=SlKbxySNeEhBLHzDcs-T-1yXBwJgK-H1Lh4Eaup7nlM