You can specify different timeouts for messages in the MTA queue based on the message priority. If the MTA cannot deliver the message within the specified time period, it returns a non-delivery report (NDR) to the sender. You can specify different timeout values for urgent (high importance), normal, and non-urgent (low importance) messages.
You specify timeouts by setting registry keys on each Microsoft Exchange Server computer you want to use timeouts on. Create the following DWORD registry keys, and then modify them to set the value of the key to the timeout in minutes.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
MSExchangeMTA\Parameters\Message timeout (urgent), minutes
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
MSExchangeMTA\Parameters\Message timeout (normal), minutes
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
MSExchangeMTA\Parameters\Message timeout (non-urgent), minutes