MyPobox supports MX and A/PTR records, for your domain (and its subdomains, if you choose to set them up.) If you would like to host other services, particularly DNS options that Pobox does not support, you will want to set up your domain at the other provider, then follow our instructions for setting up the domain's MX records to point to Pobox. No changes should be required on the Pobox end -- once your MX records at the other provider are set up to point to Pobox, your mail should begin flowing immediately.
IWantMyName offers a wider selection of top level domains (TLDs) and has an easy setup process for using their domains with Pobox.
DNSimple offers a wide selection of top level domains (TLDs) and a friendly support staff.