Selective Mail Downloading

Thanks for the absolutely fabulous apps: Portable Firefox and Portable Thunderbird. After a prolonged search, I can't find the solution to this problem. I have a 256 MB flash drive that is too small to download the very large volume of stored mail on my SMTP server. I would like to selectively download recent mail, say for the last 10 days. I can't figure how to do this in Thunderbird and can't find an extension that will allow me to designate that this is all I want. Thanks again for your help.