It looks like DynDNS removed some functionality in a site upgrade. The DynDNS instructions in the Google Apps documentation are no longer accurate because DynDNS does not allow priority values to the MX records (at least I didn’t figure it out).
Properly setting up email so that the Google Apps Dashboard doesn’t complain is super straight forward, so long as you know how. You should provide DynDNS a single MX record:
The answer is hidden in, “Other domain hosts”, step 3 of the documentation ; if you skim, you’ll miss it.