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

Results formta-sts.datetimeoffset.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2025-11-06T10:15:29.107	2025-11-06T09:16:58	2026-02-04T09:16:57	mta-sts.datetimeoffset.com	mta-sts.datetimeoffset.com	C=US, O=Let's Encrypt, CN=R13
2025-11-06T10:15:28.876	2025-11-06T09:16:58	2026-02-04T09:16:57	mta-sts.datetimeoffset.com	mta-sts.datetimeoffset.com	C=US, O=Let's Encrypt, CN=R13
2025-08-11T15:00:53.898	2025-08-11T14:02:23	2025-11-09T14:02:22	mta-sts.datetimeoffset.com	mta-sts.datetimeoffset.com	C=US, O=Let's Encrypt, CN=R10
2025-08-11T15:00:53.719	2025-08-11T14:02:23	2025-11-09T14:02:22	mta-sts.datetimeoffset.com	mta-sts.datetimeoffset.com	C=US, O=Let's Encrypt, CN=R10