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

Results formail.secretsofcardmagic.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2018-09-03T22:30:01.056	2018-09-03T21:30:00	2018-12-02T21:30:00	pebibletips.com	mail.secretsofcardmagic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-09-03T22:30:00.522	2018-09-03T21:30:00	2018-12-02T21:30:00	pebibletips.com	mail.secretsofcardmagic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-07-04T22:21:39.147	2018-07-04T21:21:38	2018-10-02T21:21:38	clientgrowth.secretsofcardmagic.com	mail.secretsofcardmagic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-07-04T22:21:38.987	2018-07-04T21:21:38	2018-10-02T21:21:38	clientgrowth.secretsofcardmagic.com	mail.secretsofcardmagic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3