Password for Mail

Yes. If you're running Leopard, go to the Parental Controls preference pane. You can restrict use of Mail.app, which will make the system require an administrator's password to open it.

If you just want to restrict getting (or sending) mail, tell Mail not to remember the password for the account.

Note, by the way, that it may be easier to create a "restricted" account that doesn't have access to whatever programs you don't want getting used, then have whoever the restricted users are use only that account. Basically a "kid" account, or whatever, with limited access.

You can then have a less restricted account for your own use so that you only need to log in once to get to your applications. Switching between accounts is quite easy using Fast User Switching (available under Login Options in the Accounts Preference Pane).

If you're using 10.4, the parental controls are somewhat less full featured (I believe they're located in the Accounts pref pane, but I don't have a computer running 10.4 handy to check).

The built-in help probably has explanations about all of this.