I got this response from support yesterday:
"We are greatly sorry for the inconvenience this 450 error has caused you.
This error came from a handful of the servers on our load balancer for a period of about 9 hours intermittently. Our admins were able to connect this to the email policy daemons connections being filled due to a spam -run through our servers . Actions have been taken and this appears to be
cleared up. We will continue to monitor the traffic level to insure long term stability. "
Things cleared up for the rest of the day, however it has started up all over again today.
Still waiting for a response to my new ticket…