First of all, it's very important you update your copy of Entourage. It
might solve this problem and might not, but it's worth doing anyway.
Download and install the Office X 10.1.2 update, free from Microsoft.
<
http://www.microsoft.com/mac/DOWNLOAD/OFFICEX/OfficeX_1012.asp>
If that doesn't work, have you checked for rogue rules (Tools -> Rules)?
You might also want to try disabling the Mailing List Manager if you use it
to see if anything in there is causing the problem.
If, after disabling rules and the Mailing List Manager, you continue to
experience the problem, this could be a sign of database corruption. Try
rebuilding your database. First, go to Entourage -> Turn Off Office
Notifications. Now, quit Entourage, and then restart Entourage holding down
the Option key. When the Rebuild dialog box comes up, choose to do a
Typical Rebuild. If that doesn't work, you should try again, doing an
Advanced Rebuild.