Sure. Set up a password for your user account, and don't let anyone else use it. Make another account for guests to use.

You're probably thinking, "But I only want to keep people out of my mail! Why do I have to keep people out of my whole account?"

Well, I'll tell you.

Suppose you locked people out of mail.app, but gave them access to your computer.

They could still:
1. Open your mailbox using another program, and read all of your mail.
2. Delete your mailboxes from the Finder, to keep you from reading your mail.
3. Delete mail.app (if you're running as admin) and keep you from reading your mail.

If it's worth protecting, it's worth really protecting. If not, then who cares?