Dyndns.org provides pretty minimal and restrictive support for setting up MX records for their free dns service. You need to pay to get more control. What they provide for free may or may not work for this.
I'm still using them for my homeip.net "domain", but for my registered .com domains, I use zoneedit.com which provides lots more function for free. -- TriLUG mailing list : http://www.trilug.org/mailman/listinfo/trilug TriLUG Organizational FAQ : http://trilug.org/faq/ TriLUG Member Services FAQ : http://members.trilug.org/services_faq/ TriLUG PGP Keyring : http://trilug.org/~chrish/trilug.asc
