if you are on a BES it's likely a corporate policy to keep messages for only 30 days, do not try to remove the IT policy from the device if you are on a BES.
The *only* way to get that changed is to talk to your BES admin, and see if it is possible.
BES 5.0.2 MR4
Exchange 2010 SP1 RU2
WES 2008 Alumni