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

Results forwebmail.silverback.cl

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-10-18T08:45:27.098	2019-10-18T07:45:25	2020-01-16T07:45:25	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-10-18T08:45:25.347	2019-10-18T07:45:25	2020-01-16T07:45:25	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-18T09:19:43.191	2019-08-18T08:19:41	2019-11-16T08:19:41	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-18T09:19:41.909	2019-08-18T08:19:41	2019-11-16T08:19:41	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-18T09:02:44.093	2019-06-18T08:02:43	2019-09-16T08:02:43	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-18T09:02:43.397	2019-06-18T08:02:43	2019-09-16T08:02:43	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-04-18T08:56:27.079	2019-04-18T07:56:26	2019-07-17T07:56:26	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-04-18T08:56:26.291	2019-04-18T07:56:26	2019-07-17T07:56:26	silverback.cl	webmail.silverback.cl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3