The server your site sends mail through has no RDNS 
We know. We’ve been arguing with our colo about it for a little bit (they say they’ve delegated it, but they haven’t). Apologies for the inconvenience or missed mail. We’ll hopefully have it sorted soon.
I guess this is the same colo facility at which you lost connectivity for half a day?
Scary.
It is, and it is problematic. We’ve been with them for years, and have never had problems until recently. They are under new ownership about a year ago, but service remained good and reliable. I don’t necessarily consider this a deal-breaker situation, but it is somewhat worrying. The delegation issue is, I think, merely a communication problem. I put in the request after hours, without realizing it can only happen during business hours (while updating individual reverse records can happen anytime). There was some back and forth, and they assigned the name, but then when it came time to delegate the whole zone, something seems to have gone awry.