Masking email fields in UAT instance

Hi,

I have a UAT instance of Dynamics 365 online that I need to mask data in certain entities. I have managed to do bulk updates using Bulk Data Updater in XrmToolBox and made changes to annotation, appointment, task, incident and opportunity. But when I try to change any fields in the email entity it fails. I want to change the subject, description, to and from fields, but it seems not possible.

Is it possible to change these fields or some of them? How can it be done?

Regards
Frode