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

Results formail.miningground.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-05-10T05:54:02.932	2019-05-10T00:00:00	2019-08-08T23:59:59	miningground.com	mail.miningground.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2019-05-10T05:53:44.589	2019-05-10T00:00:00	2019-08-08T23:59:59	miningground.com	mail.miningground.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2019-04-24T05:09:41.993	2019-04-24T00:00:00	2019-07-23T23:59:59	miningground.com	mail.miningground.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2019-04-24T05:09:24.199	2019-04-24T00:00:00	2019-07-23T23:59:59	miningground.com	mail.miningground.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"