First Verify that your mailbox is NOT full (so the full mailbox notification is incorrect), you should do the following:
- Open Outlook, click File (left top), Click Account Settings, double click your account name, and then uncheck “Use Cached Exchange Mode” and finish the wizard
- Close Outlook. Delete your OST file. The default location for you OST-file on Windows 7 is C:\Users\user\AppData\Local\Microsoft\Outlook\. Either copy and paste the path in Windows Explorer, OR Enable Hidden Folders to view the AppData folder.
- Open Outlook again and enable Cached Exchange Mode again (like you did in step 1, but now recheck)