Internet Email Inbox Truncated

I have a Pearl 8100 from TMobile with BES installed. In addition, I have enrolled 3 Yahoo EMail Accounts via BIS. Lately I notice that I can only access emails pushed to my Blackberry in the same day and all the old emails pushed days ago no longer show up. Is this a device memory issue or corrupted OS issue?

Please advise.

on an 8100 it's most likely memory issue.

I know I can set memory size for media files and cache size for email attachments. But how can I find out how much memory is left? More importantly, how do I cleanse the memory?

Thanks very much!