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

Results formx0.mail.marketmatrix.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2025-07-19T09:36:25.895	2025-07-19T00:00:00	2026-07-18T23:59:59	mx0.mail.marketmatrix.com	mx0.mail.marketmatrix.com	C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G2
2025-07-19T09:36:18.399	2025-07-19T00:00:00	2026-07-18T23:59:59	mx0.mail.marketmatrix.com	mx0.mail.marketmatrix.com	C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G2
2021-01-30T11:56:39.463	2021-01-30T00:00:00	2022-01-30T23:59:59	mx0.mail.marketmatrix.com	mx0.mail.marketmatrix.com	C=US, O=DigiCert Inc, OU=www.digicert.com, CN=Encryption Everywhere DV TLS CA - G1