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

Results formail.intrior.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-06-27T09:13:49.087	2019-06-27T08:13:48	2019-09-25T08:13:48	intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-27T09:13:48.663	2019-06-27T08:13:48	2019-09-25T08:13:48	intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-22T05:46:21.9	2019-03-22T04:46:20	2019-06-20T04:46:20	intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-22T05:46:20.597	2019-03-22T04:46:20	2019-06-20T04:46:20	intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-22T05:39:13.872	2019-03-22T04:39:12	2019-06-20T04:39:12	www.intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-22T05:39:12.905	2019-03-22T04:39:12	2019-06-20T04:39:12	www.intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-22T05:33:40.545	2019-03-22T04:33:39	2019-06-20T04:33:39	www.intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-22T05:33:39.385	2019-03-22T04:33:39	2019-06-20T04:33:39	www.intrior.com	mail.intrior.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3