Beware: Mimecast Plugin does not have the same functionality in Outlook 2016 that it did in previous versions.
If your users are use to using the reading preview function in Outlook, it will not work in Outlook 2016 with Mimecast stubbed messages. You will need to either change your user experience or stop stubbing your messages. This issues was reported to Mimecast when Outlook 2016 was first released to the public but Mimecast has only recently started "investigating" the issue. I hope we get a fix or enhancement to the plugin before Outlook 201X is released.
This has caused allot of issue with our users.