It can be anything! An apex, subdomain, or even a URL.
Domain Name: email.email-1-staging.circle.soDomain Status: No Object Found>>> Last update of WHOIS database: 2024-09-22T07:30:05.592Z <<<
Domain Name: email.email-1-staging.circle.so
Domain Status: No Object Found
>>> Last update of WHOIS database: 2024-09-22T07:30:05.592Z <<<