Blocking port 25 outgoing on Tp-link router

I have a site with a tp link 8960 adsl router that has been blacklisted for spamming.To avoid it happening again I'd like to block port 25 to all but the internal IP of one device(copier). Can't seem to figure it out on this router, has anyone else ever done this? And I've already done a clean, just wanted to add an extra layer of security.