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

Results forwww.mail.whm11-test.dev.mailchannels.net

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-04-04T18:35:13.777	2019-04-04T00:00:00	2019-07-03T23:59:59	whm11-test.dev.mailchannels.net	mail.whm11-test.dev.mailchannels.net	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2019-04-04T18:34:59.226	2019-04-04T00:00:00	2019-07-03T23:59:59	whm11-test.dev.mailchannels.net	mail.whm11-test.dev.mailchannels.net	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2019-03-23T00:25:34.852	2019-03-23T00:00:00	2019-06-21T23:59:59	whm11-test.dev.mailchannels.net	mail.whm11-test.dev.mailchannels.net	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2019-03-23T00:25:19.305	2019-03-23T00:00:00	2019-06-21T23:59:59	whm11-test.dev.mailchannels.net	mail.whm11-test.dev.mailchannels.net	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"