There are many email archiving solutions which accomplish this if you have the money for them. In our organization we use an archiving solution and it copies the entire mailbox every day as well as strips messages of a specific age from the exchange mail store. The advantage is the archiving server uses true single instance storage for all the messages, saving valuable storage space. Users can always find their old messages on the web page for the archiving software and they can not delete anything from that archive server. That’s one solution anyway.