Password protecting Mail accounts

There isn't a way. Set up separate user accounts for your users.

System Preferences, Users & Groups.

Make sure you log out of your account when you step away.

There are various settings you can use in the security system Prefs to have it lock the screen on sleep and after idle time.

You can create an encrypted disk with disk utility for the mail app, so that each time you click on it it asks for a password.

1. Open Disk Utility (located in /Applications/Utilities/).
2. Click the New Image button, or choose File > New > Blank Disk Image.
3. Type a name in the Save As field (like mail). This name will be used for the disk image (.dmg) file.
4. Change the save destination if you wish.
5. Select a size for the disk image file from the Size pop-up menu. Find out your current mail app size (Open Finder - Applications - Ctrl+click Mail - Get Info) then select a size slightly larger than that.
6. Choose a different volume format if you don't want to use the default Mac OS X Extended (Journaled).
7. Choose an image format. You can use "sparse disk image" for a disk image that only uses as much space as it needs, rather than a set amount of space. If you're not sure, use "read/write disk image" choice.
8. Choose 128-bit AES encryption (and/or 256-bit AES in Mac OS X v10.5 or later) from the Encryption pop-up menu to encrypt the image's contents with a password. If you don't choose an encryption, your new image won't be encrypted.
9. Click the Create button.
10. Enter and verify a good password in the dialog window that appears. This password will be saved in your keychain by default, or you can deselect "Remember password (add to keychain)" if you don't want it saved. You can store the password in the keychain for convenience.
11. Click OK.
12. The disk image should be on your desktop. It looks like this.
13. Double click on it, to open.
14. Open your mail app in Finder, click and drag it to the new disk image.
15. From this window, drag the mail app onto your dock (if you want it accessible from the dock, too.)
16. Close the window and eject the disk. (Ctrl+click the disk image - Eject)
17. If you click on the Mail app on your dock, it should prompt you for a password. Type the password you created for the disk image. Voila, you've just password protected your mail.
18. You will need to eject the disk image after every use for mail app to ask for a password again. If you do not do this, mail will be opened and accessible to anyone.

Important: If you forget the password, data stored in the encrypted disk image cannot be retrieved. If you have saved the password in the keychain, the password will be available to you there.

I have been used MacFort to protect Mail over two years, works perfectly with Mavericks,

See:http://www.madowsoft.com/how-to-password-protect-mac-mail.html ,Hope can help you