Firewall - Network Services Filter

[MikeMo]

Hello,

A recent spam-bot outbreak from inside my network has encouraged me to lock down outgoing port 25 on my RT-AC66U to all client computers, however I need to leave it open for my Exchange Server. There doesn't seem to be a way to enter a range in the Source IP field in the Network Services Filter page. Do I need to list all other internal IP addresses other than the server as separate entries?

Does anyone have any suggestions?

Thanks,
Mike