Some common reasons are: 1. The size of your email box has become too large. To fix this problem you need to delete the items in your "Sent" folder and "Deleted Items" folder. If you do not want to delete these items, then you will need to create a personal folder in which to store them. 2. Cached Exchange Mode is set to "On." To fix this issue, you will need to turn it "Off." To turn Cached Exchange Mode off, open Microsoft Outlook, click on "Tools" in the menu bar, and choose "Email Accounts." Once the dialogue box opens, choose the "Next" button, then choose the "Change" button, and then you should find the check box marked Cached Exchange Mode underneath the Microsoft Exchange Server Name. All you need to do is deselect the check mark from the box, click "OK" when the message box appears indicating that you must restart outlook for the change to take affect, choose the "Next" button, select "Finish," and then close and reopen Microsoft Outlook.