That would be the TTL 3600 second (1hr) retry setting for the email queue. Depending on the config, it may do that for about 6 hrs or so before quitting and sending a fail message to the sender.
Not a big deal, just that MX'ers weren't available. It was part of the DNS not being propagated. More specifically, it was because it wasn't configured correctly to propagate expeditiously. May well have been because the domain registration was the limitation rather than the TTL. Just a guess, I don't know that 100%.