Any customer with the Continuity service as part of their Mimecast solution will have access to the Mimecast Personal Portal.
https://webmail-uk.mimecast.com/u/login/?gta=portal#/login
In the event of a customer mail server outage, users can log in to the Mimecast Personal Portal and continue to send and receive emails via an easy to use web interface.
Active Directory Sync means that your users’ credentials are already stored on the Mimecast server, so they just need to use their domain email address and password.
Once logged in, this is what the interface looks like:
The user can even view held and bounced emails:
