Brian, It sounds to me like you need to change the setting of your email program (Outlook or ThunderBird whatever) to Leave Messages on Server.
If you don't leave the messages on the server, you can't get them from the BB. Chances are you can google it to find the exact steps to turn that on. I prefer mine to only go one place depending on where I am so that my Memory doesn't get filled with useless crap I have already replied to.