Pingdom suck. As an example I fail with sending emails to email@example.com. Just had 6 people on 6 different ISPs (in the UK) send an email to the email address in my SOA and I received everyone of them.
The warnings are just reverse lookups when using IPv6. That *could* be GoDaddys issue if they are not IPv6 ready. Ignore the reverse lookups for now, that is mainly used by security conscious persons, as long as the address resolves to an IPv6/IPv4 address then all is well. (GoDaddy may not be designated as the master for the address range in which your reverse lookups happen, my site fails as well on pingdom but passes on others).
The biggest use in reverse lookups as they work now is for checking if a mail server is really who they say they are. In order for it to work you have to have a static ip that's just yours and whomever provides that IP to you has to set up a PTR record for it. You can request it from your ISP but unless you host your own mail off that IP I wouldn't worry about it.
dnsstuff is great, and worth the money if you have to use it daily, that said .. im just a simple consumer who needs it once a year .. it's not worth the money. My subscription run out. I invested in it once. And used it on all my domains. gdamn 4 months later I changed hosts and had to do it all again, but it expired.