how do i retrieve old email from pop server?

[breadbin]

Hi, I was wondering if anyone can tell me how to download all my mail from my pop account and not just the new mail. I have been using a web based pop account and now I have my computer set up with Mozilla Thunderbird and thought it would be straightforward to download all my mail onto my computer and delete it from the server. Thunderbird just checks for new mail and won't do anything with the old ones. From what I can remember outlook was the same way too.