A recent problem I had with receiving alerts in my inbox was due to the fact that I am using the Outlook 2007 and the Exchange is still the 2003 version.
This is a known issue and there is a hotfix to solve this, you can find more info here: http://support.microsoft.com/kb/930807
The big pain in this is that you need to contact the Microsoft support in order to have the hotfix . . .
Nevertheless there is a workaround to this, and it has to do with the usage of the Exchange Cache in your Outlook client.
So trying to guide you to the option:
Option -> E-mail accounts -> change ->more settings -> advanced: uncheck the “Use Cached Exchange Mode”
Note: This might be a poor approach and remember it should be used has a workaroud until your System Admin installs the hotfix.