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

Results forpost.xero.com

Resolver

Location

MX

Name	TTL	Value
post.xero.com	3600	10 mxa.mailgun.org
post.xero.com	3600	10 mxb.mailgun.org

Name	TTL	Value
post.xero.com	3600	google-site-verification=XRqrdl0RRC8SUrgJkvq0iUm1wMLdCIx87fPNt6OPL9s
post.xero.com	3600	google-site-verification=uq3EiTWH6vQSmWGdP66TXfQXbBY_G7NjypAdRGjMGOQ
post.xero.com	3600	v=spf1 mx include:mailgun.org ~all
post.xero.com	3600	yahoo-verification-key=f79TmSflTlCJ9MYWjk3s4ocW8yLtr6LL6bLJ0KZc4AY=