Preventing Out of Office Messages from being sent ou to distribution lists
http://support.microsoft.com/?kbid=825370
Valid for Exchange 2003 only.
Key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
Value: SuppressOOFsToDistributionLists (DWORD)
Set 1 to suppress or 0 to allow.
The setting should be set on mailbox servers.
The setting prevents sending OOFs unless the the recipient is in the TO,CC or BCC(if no one other recipients
appear in the other fields).