On the Mimecast portal, navigate to Monitoring > URL Protection. Here you will see the URLs that have been scanned by Mimecast, how they have been categorised, the sending address, recipient, and whether they have been overridden, if Mimecast has deemed as malicious.
You can search by sender/recipient address, or URL.
Click on the URL log entry in question, and choose the appropriate option; Report, Allow URL, or Block URL.
Additionally, you can also Report the URL to Mimecast, if malicious.