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

Results forwebmail.checkoutbutton.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-02-03T03:09:46.555	2019-02-03T02:09:45	2019-05-04T02:09:45	viraledia.com	webmail.checkoutbutton.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-02-03T03:09:45.748	2019-02-03T02:09:45	2019-05-04T02:09:45	viraledia.com	webmail.checkoutbutton.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-03T00:23:52.112	2019-01-02T23:23:51	2019-04-02T23:23:51	checkoutbutton.com	webmail.checkoutbutton.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-03T00:23:51.469	2019-01-02T23:23:51	2019-04-02T23:23:51	checkoutbutton.com	webmail.checkoutbutton.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-11-03T01:03:16.384	2018-11-03T00:03:15	2019-02-01T00:03:15	checkoutbutton.com	webmail.checkoutbutton.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-11-03T01:03:15.142	2018-11-03T00:03:15	2019-02-01T00:03:15	checkoutbutton.com	webmail.checkoutbutton.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2017-05-23T18:31:07.816	2017-05-18T00:00:00	2017-08-16T23:59:59	checkoutbutton.com	webmail.checkoutbutton.com	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"