DynDNS is currently undergoing a massive DDOS attack. Sites that outsource their DNS to DynDNS are currently difficult to reach.
I have found that Google’s name servers tend to cache data forever and so even when the primary name server is down, Google’s name servers will usually return good data.
I had quite a bit of mail backing up in the mail queues of mx1 and mx2 owing to the inability to resolve DNS of destination sites. I have added a Google public name server to these machines and re-queued the mail and much of it is making it out now.