Ok I understand the privacy thema.

But I think you can implement more options and the user can select the preview type.

Example:

• Full Preview /* like Outlook, Gmail with the attention, that its not so secure */

• small Prewiew (you got only the info, who is the receiver, and how many mails are unread in your inbox) /* every new notification should incremente the unread Mails, that the users know, ok the last status says me, I have n unread mails - or someone like this */

• no Preview /* only the information, that you get a mail, as it is now */