Popcorn is an ultra-lightweight POP3/SMTP e-mail client. Being a true client/server application, it does not work in the same way as a typical mail program. This means that you're reading mail from the POP3 server directly, without downloading it to a local mailbox on your PC (you can save mail messages locally if you want to).
More info:
hxxp://www.ultrafunk.com/products/popcorn/