Wehn you say second mailbox what do you mean

could be

a. a second mailbox in your existing profile or
b. a second profile with a different mailbopx account setup in it

in either case you can indeed password protect an email account

if you are using exchange then i suggest using a different profile for
you second account this is because you cannot use seperte inbox when in
exchange mode, your pop or imap accounts will receive mail to the
exchange account inbox. Having seperate profiles negates this issue.

hope this helped