If you want to send an e-mail Privately it is quite simple and if someone else has access to your e-mail account from their computer, they cannot see the e-mails that are marked Private in either your Inbox or Sent items.
This would most likely be the case in the boss/assistant relationship as many assistants are given access to their boss's e-mail account.
File New Message > Options button on the toolbar (or View / Options).
Message Settings / Sensitivity - change to Private. The same process needs to be repeated for each e-mail sent.
The person receiving the e-mail will see "Please treat this as private".
I thought it was a handy trick to know. You never know when your boss might ask if this can be done.