We were experiencing significant delays with delivering outgoing email email to domains hosted on Mimecast.
The Mimecast status page reported the following 5 November:
….we are still seeing intermittent connection issues causing mail processing and delivery delays for customers on the ZA grid. We believe this connection issue is being caused by an unusual volume of emails attempting delivery and causing contention issues within our environment. This means some emails are being automatically retried because the service they’re attempting to reach is intermittently busy and unable to respond promptly. We are seeing these emails correctly deliver on a later retry.