I have seen this before and Microsoft has a fix for it here. You can read about it at this link. They will include this hotfix in their next service pack. But if you want it before then contact them via this link.
Microsoft goes on to say:
. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, Microsoft recommends that you wait for the next Microsoft Exchange Server 2003 service pack that contains this hotfix.