I think the OP *wants* to have password control instead of remembering passwords?
I don't think there's an easy way to do exactly what you want. It's been discussed a few times before, and the big problem is that the whole system is based on a account-level access privilege. You're expected to lock the screen or go to the login window if you want to prevent access.
Some options you might explore....
- I guess in principle you could put the Mail.app in a password-protected disk image. This would be "fake" protection as all the mail would be accessible in text format from the command line.
- You *can* set it up so that you're asked for the password of each / certain accounts each time you check it. To do this, you delete the password from the account pane of the preferences, and then when it pops up and asks you for it, you make sure that the remember button is not selected. Then it should ask again the next time it tries. But this will not protect access to mail already on the computer. Also if it's an IMAP mailbox, you might be able to set the "keep offline copies" option in the advanced tab of the accounts prefs to be no messages kept. Then when you're not logged into it, you should see an empty inbox.
- Otherwise you should explore a solution that involves using account-level privileges. For instance, you can set up a guest account that has no access to your mail, etc, but has a web browser and other things you want. Then you fast user switch to the account with the mail when you need it. Or just lock the computer when you're not using it.