Empty "To:" field when receiving mail

[Jacobs]

Dear Microsoft and engineers,

Lately we have a strange problem on our Exchange server.
When a user is receiving a mail the field "To:" (dutch: "Aan:") is empty.
This problem occours on both Outlook 2003 and the Web Access panel.
It also occours on all users (including newly created users).

Anyone got any clue where we should seek to resolve this problem?
We are using Microsoft Exchange 2003 (with all updates including SP2).
This is integrated in our Small Business Server 2003.

Screenshot of the problem:

http://www.digithost.nl/files/exchangeproblem.JPG

Thanks in advance.