TBP - Password

Is there anyway to set up Thunderbird where you can't read existing mail that is in the program with out the password being entered.

Basically, when I click on TBP, I had it set to where I had to enter my password every time I opened Thunderbird. Earlier I was wondering what happens if you click on cancel and you don't enter the password, could someone still read the existing email that has already been downloaded. I found out that it doesn't block you from see current email, it just won't download new email.

Thanks!