dnsmx is part of a set of user utilities provided by djbns.
Lookup MX records of a domain:
dnsmx beandog.org
10 eforward2.registrar-servers.com
10 eforward1.registrar-servers.com
15 eforward4.registrar-servers.com
10 eforward3.registrar-servers.com
20 eforward5.registrar-servers.com