I don't know of a way to limit a mailbox, but if you go into messaging then securtiy administratior, you can set-up how often the every mailbox needs the password changed. I believe 364 days is the most you can put in, and i don't think 0 will work, but you could try. i am not at work, so i can't try and see if 0 will work. i hope this helps.
Scott Williams
IT, United States Coast Guard.