If your suddenly unable to receive email and your senders are getting bounces that look like this:
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 unrouteable address (state 14).
...Then you might check to see who is hosting your domain name. If it's MyDomain[.]com, you are in good company. There are many of us experiencing an outage right now due to a corrupt/malicious/broken zone file.
It is my understanding that the hosting company is aware of the issue and is working on a resolution. In my case, there is a fancy new MX record that happens to have a priority of 0 that I suspect is causing the issue (m1[dot]dnsix[dot]com). I originally suspected Google Hosting Services (sorry Google) but I think I have cleared them through investigation as all fingers are now pointing to MyDomain.
In the meantime...email just ain't happening. Ugh.