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

Results formailgw.mail.marketmatrix.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2025-03-01T07:13:03.085	2025-03-01T00:00:00	2026-02-28T23:59:59	mailgw.mail.marketmatrix.com	mailgw.mail.marketmatrix.com	C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G2
2025-03-01T07:12:34.102	2025-03-01T00:00:00	2026-02-28T23:59:59	mailgw.mail.marketmatrix.com	mailgw.mail.marketmatrix.com	C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G2
2021-01-29T09:20:12.232	2021-01-29T00:00:00	2022-01-29T23:59:59	mailgw.mail.marketmatrix.com	mailgw.mail.marketmatrix.com	C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G1