IPhone 4 taking mail off of server after receiving?

My mom just got herself an iphone 4 and she expects me to set it up for her, her mail and contacts and stuff, so I added her AOL mail to the phone but when she checks it on her phone it appears to take it off of the AOL server because when she logs into her AOL account from a computer the message that she checked on her phone is not in her inbox at all. I have a 3GS iphone but i use yahoo mail and I have never had a problem like this. Does anyone know how to solve this problem?