I note the author details reference a version that does not yet exist..
(https://splunkbase.splunk.com/apps/#/author/mimecast_4_splunk_integration) as there are number of known CIM compatibility and field extraction issues we all hope that Mimecast would consider making this a public code project so members of the community to contribute directly.
The Mimecast App is provided by Mimecast directly - you should try reaching out to your Mimecast account manager and ask if they have any details or can find out.
