I'm having a similar problem with "Content Hidden" with the Galaxy S7 email app provided pre-installed with the phone:
1) setting up a gmail account in the samsung email app - will show the preview in the notification bar. Just like my S5 did.
2) setting up my Exchange Active Sync causes all the samsung email app to switch to "content hidden" for both the existing gmail account notifications and the new Exchange Active Sync account i just added.
3) disabling the device administrator allows the previews to return but also disables my Exchange Active Sync email so it's not really a solution.
I talked to our exchange admins and they haven't changed anything. it seems to be something on the S7 or Marshmallow is misinterpreting the device admin settings pushed down. My S5 did not behave like this.
The kicker is - that the standard email app is really the most desirable because it syncs contacts better than the Outlook app for android. It also a double whammy because my Gear S2 is USELESS when it comes to emails due to this "content hidden" thing that happens. The Gear S2 returns "security does not allow..." when i try to read the message.
How do I get to the details of the device admin settings that are pushed to the device? I wish I had enough detail to articulate the exact issue to my exchange team. Our Exchange server is older so I suspect that in the absence of certain details Samsung / Marshmallow is flipping certain settings by default.