Creating MX records for a subdomain

I’d like to be able to assign users either a second-level domain ( email address or a third-level domain ( email address, so I need to add the necessary MX records for (I think). I can’t figure out how to use the interface to do this, since the form for creating truly custom DNS entries doesn’t have MX records as an option, and the custom MX record form doesn’t have a way to assign them to subdomains. Has anybody figured out a way to do this? Thanks!

Create a Parked domain for what you want. Then you’ll be able to add email addresses for that subdomain and that, in turn, will set up the mail entries in DNS.