First things first : are you sure that the ACL on your mail file does not include people it should not ? To check, in your mail go to File/Database/Access Control and review the people entries (not server entries) that are in there.
If there is anything that seems suspicious to you, call your administrator and have him explain the how and why.
Next, how exactly can you be sure that people are reading your mail ? If you send without saving, then they are not reading it from you, but from the recipient(s). You have no control over that.
As for recieving email, only the recipients you designate can. There can be no "unauthorized" recipients. Again, the recipients you designate can also forward the mail, and you have no control over that, unless you are using Notes 5x or 6x. If so, you can specify Do Not Copy in the Delivery Options. That will restrict the recipients ability to easily forward your mail.
Pascal.