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

Results foremail.goldentrailer.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-01-04T14:02:29.378	2020-01-04T00:00:00	2020-10-09T12:00:00	email.goldentrailer.com	email.goldentrailer.com	C=US, ST=CA, L=San Francisco, O="CloudFlare, Inc.", CN=CloudFlare Inc ECC CA-2
2020-01-04T14:02:28.262	2020-01-04T00:00:00	2020-10-14T12:00:00	email.goldentrailer.com	email.goldentrailer.com	C=US, ST=CA, L=San Francisco, O="CloudFlare, Inc.", CN=CloudFlare Inc RSA CA-1
2019-03-30T15:59:51.142	2019-02-04T00:00:00	2020-02-04T12:00:00	email.goldentrailer.com	email.goldentrailer.com	C=US, ST=CA, L=San Francisco, O="CloudFlare, Inc.", CN=CloudFlare Inc ECC CA-2
2019-02-04T21:10:06.594	2019-02-04T00:00:00	2020-02-04T12:00:00	email.goldentrailer.com	email.goldentrailer.com	C=US, ST=CA, L=San Francisco, O="CloudFlare, Inc.", CN=CloudFlare Inc ECC CA-2
2019-02-04T21:10:05.176	2019-02-04T00:00:00	2020-02-04T12:00:00	email.goldentrailer.com	email.goldentrailer.com	C=US, ST=CA, L=San Francisco, O="CloudFlare, Inc.", CN=CloudFlare Inc RSA CA-1