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

Results formail.rivetsolution.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2022-03-22T21:41:17.391	2022-03-22T00:00:00	2022-06-20T23:59:59	rivetsolution.com	mail.rivetsolution.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2022-03-22T21:41:16.407	2022-03-22T00:00:00	2022-06-20T23:59:59	rivetsolution.com	mail.rivetsolution.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2022-03-22T19:42:13.354	2022-03-22T00:00:00	2022-06-20T23:59:59	rivetsolution.com	mail.rivetsolution.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2022-03-22T19:42:12.755	2022-03-22T00:00:00	2022-06-20T23:59:59	rivetsolution.com	mail.rivetsolution.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"