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

Results formx.hubapi.com

Resolver

Location

A

Name	TTL	Value
mx.hubapi.com	120	3.214.238.166AWS EC2 (us-east-1) / Amazon Technologies Inc.
mx.hubapi.com	120	35.171.192.19AWS EC2 (us-east-1) / Amazon.com, Inc.
mx.hubapi.com	120	52.1.1.25AWS EC2 (us-east-1) / Amazon.com, Inc.