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

Results formail.routerstack.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2025-03-04T19:04:25.742	2025-03-04T18:02:14	2025-06-02T19:00:22	routerstack.com	*.routerstack.com	C=US, O=Google Trust Services, CN=WE1
2025-03-04T19:02:44.317	2025-03-04T18:04:13	2025-06-02T18:04:12	routerstack.com	*.routerstack.com	C=US, O=Let's Encrypt, CN=E6
2025-03-04T19:02:44.013	2025-03-04T18:04:13	2025-06-02T18:04:12	routerstack.com	*.routerstack.com	C=US, O=Let's Encrypt, CN=E6
2025-03-04T19:02:14.373	2025-03-04T18:02:14	2025-06-02T19:00:22	routerstack.com	*.routerstack.com	C=US, O=Google Trust Services, CN=WE1
2021-06-04T07:42:54.446	2021-06-04T00:00:00	2021-09-02T23:59:59	mail.routerstack.com	mail.routerstack.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2021-06-04T07:40:34.407	2021-06-04T00:00:00	2021-09-02T23:59:59	mail.routerstack.com	mail.routerstack.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"