Ik heb wat gevonden op :
http://support.microsoft.com/kb/818222/
Ik ga dit om half 5 uitproberen als alle clients uitgelogd zijn.
Method 1: Configure the XEXCH50 Registry Subkey
In Exchange Server 2003 and Exchange Server 2000 Service Pack 3 (SP3), you can suppress the sending of the XEXCH50 command to external domains. To do so, follow these steps.
Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SMTPSVC\XEXCH50
Note If the XEXCH50 registry subkey is not present, create it. To do this, point to New on the Edit menu, and then click Key. In the New Key #1 box, type XEXCH50, and then press ENTER.
3. Right-click XEXCH50, point to New, and then click DWORD Value.
4. In the New Value #1 box, type SuppressExternal, and then press ENTER.
5. Right-click SuppressExternal, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. Quit Registry Editor.