Vista - windows mail, how to hide e mails until user sign in

Using windows mail to access Comcast mail I set the account to require a
password. That stops anyone else from receiving new messages.

But all the old messages are right there for anyone to view as soon as
windows mail is opened?

What do I do so the old e mails are not visible to everyone. . . . It worked
like that in Outlook Express; had to sign in to an identity to see new and
old messages. Only difference was I had three identities then, now just the
one identity.

Thanks

Pete