Maybe I didn't explain what I'm trying to do correctly. My weak area is MX records. I understand how A records work and have a basic understanding of how DH hosts DNS stuff.
I have a fully hosted domain on DH. example.com GoogleApps is handling all the email for example.com, DH is handling the web hosting and DNS for the domain.
I have a workgroup server in my office that I would like to have publicly available via sub.example.com, and I do have a genuine static IP from my ISP. Until now I have had an A record pointing sub.example.com to my static IP and my office server and it's been fine. What I would like to add is mail serving on my office server, to host email addresses on my office server (such as email@example.com) and I understand I need an MX record to point to the server.
How do I setup an MX record on DH that points mail traffic for my subdomain sub.example.com to my office server? Does an A record point any/everything for sub.example.com to the IP associated with it, including mail/MX? This is the missing link in my brain.