BB Tour 9630 :: Email Messages Disappearing From Phone

I currently have my BlackBerry Tour configured for three seperate email accounts. The first two are working just fine. I just noticed this this morning that for the third, messages that I have read appear to be disappearing from the device. I've checked in Options and it appears that I have plenty of available space (101680072 bytes free) unless I am misreading what that number represents.

For example, I've read and responded to probably 10 emails from this account today. The emails that I read early this morning no longer show in the inbox, however the sent item for the response that I sent still does. The only items that still appear in the inbox are items that have arrived in the last 2 - 3 hours, anything prior to that, including ALL emails that I have received since setting up the account a few weeks ago are gone. The only thing that is visible for those dates are my sent items.

I had seen another thread in which someone had stated that rebooting by removing the battery might correct the issue but I tried that and it did not. I've included my settings for the email account in question below. I don't use any type of third party sync application, what the heck is going on here and how do I fix it?

General Options
Display Time - Yes
Display Name - Yes
Display Order - Name, Subject
Display Message Header on - 2 lines
Display Message Count - Unread
Display New Message Indicator - Yes
Hide Read Filed Messages - No
SMS and Email Inboxes - Theme Controlled
Seperators - Stripes

Actions
Make PIN messages Level 1 - Yes
Auto More - Yes
Confirm Delete - Yes
Confirm Mark Prior Opened - Yes
Auto Open Newest Message After Removing Device From Holster - After Sound or Vibration
Keep Messages - Forever

Email Settings
Enable HTML Mail - Yes
Download Images Automatically - Yes
Confirm Delivery - No
Confirm Read - No
Send Read Receipts - No

Email Reconciliation
Delete on - Prompt
Wireless Reconcile - Mailbox Wins
On Conflicts - Mailbox wins