Go to the mail tab, select Inbox, then Menu -> Tools -> Options (may have to scroll the list for some of these items) then Send/Receive Schedule. For the "Download messages" drop-down option do you have Today's messages in there? That means it will only pull each days messages and drop any that are older. Change this to show you whatever amount you wish such as 3 days, 1 week, 2 weeks, etc. Hope that helps.
[Carrier: Verizon Wireless] [Phone: XV6975 (HTC Imagio)] [ROM: Energy 3.13.10 Cookie Home Tab R2]
[WM Build: 23544] [Radio: 2.25.00WVL]