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

Results forwebmail.test.sysalpine.de

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-03-15T23:41:59.919	2019-03-15T22:41:59	2019-06-13T22:41:59	webmail.test.sysalpine.de	webmail.test.sysalpine.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-15T23:41:59.051	2019-03-15T22:41:59	2019-06-13T22:41:59	webmail.test.sysalpine.de	webmail.test.sysalpine.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-07T23:46:07.436	2019-03-07T22:46:06	2019-06-05T22:46:06	webmail.test.sysalpine.de	webmail.test.sysalpine.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-07T23:46:06.287	2019-03-07T22:46:06	2019-06-05T22:46:06	webmail.test.sysalpine.de	webmail.test.sysalpine.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-07T12:48:33.689	2019-01-07T11:48:33	2019-04-07T11:48:33	webmail.test.sysalpine.de	webmail.test.sysalpine.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-07T12:48:33.062	2019-01-07T11:48:33	2019-04-07T11:48:33	webmail.test.sysalpine.de	webmail.test.sysalpine.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3