Mimecast cannot deliver mail from certain domains directly to Microsoft.

We've now resolved the incident. Thanks for your patience.

The workaround seems to work. Naturally, we will continue to monitor the mail flow closely.

Mimecast says they have configured a workaround. We are testing that now. As soon as we can confirm that, we will let you know here.

Mimecast has been informed of this by us last Friday. The ticket has been escalated and we are waiting for a response. Emails are currently in the queue. If they are not delivered at all, you will receive a message from Mimecast. Until then, we assume that the messages will still be sent, but with a long delay.

Microsoft reports a '4.7.500 Server busy'.