We are struggling with the same thing here. Would be really nice if Mimecast would comment on this. We are writing Powershell scripts to automatically update AD attributes for this kind of stuff...it works, but it's more work and overhead than it should be if Mimecast would correct this.
With Mimecast Stationery functionality, it's possible to obtain incorrect results when creating and applying a Stationery Layout. Although Mimecast Support can be available to assist customers with queries, this page outlines common problems, and how Administrators can resolve them. Mimecast is unable to support HTML coding of your StationeryPreviewHide preview