Thunderbird downloading email messages

i'm just curious, how can i make my thunderbird NOT re-download messages that are already in my list?

let's say i have 500 email messages (all read), in the first install of thunderbird, it downloads them. then i close the program.

after opening it again, it re-downloads the SAME 500 messages. how to make it download just the new ones?

thanks!

I believe you just go: Tools >> Account Settings >> "Your Account" >> Server Settings

and under that heading should be all logical check boxes such as the "download new messages" thing.