We have put measures in place to work around Mimecast's issue.

A fix has been implemented and we are monitoring the results.

We are seeing some delays with emails sent to Mimecast users. It appears Mimecast is sometimes incorrectly producing the error "451 Open relay not allowed".

https://community.mimecast.com/docs/DOC-1369#451

We are investigating the issue urgently. However, we would also recommend that recipients contact Mimecast to report the issue as well.

Thank you.