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

Results formail.promo.nic.ec

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-10-13T05:10:38.896	2020-10-13T00:00:00	2021-01-11T23:59:59	promo.nic.ec	mail.promo.nic.ec	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2020-10-13T05:10:38.284	2020-10-13T00:00:00	2021-01-11T23:59:59	promo.nic.ec	mail.promo.nic.ec	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2020-07-29T20:44:55.019	2020-07-29T00:00:00	2020-10-27T23:59:59	promo.nic.ec	mail.promo.nic.ec	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"
2020-07-29T20:44:54.568	2020-07-29T00:00:00	2020-10-27T23:59:59	promo.nic.ec	mail.promo.nic.ec	C=US, ST=TX, L=Houston, O="cPanel, Inc.", CN="cPanel, Inc. Certification Authority"