What software blocks spam from open relays?
If your open relay is found by a blacklist organisation (usually if someone reports spam coming from your mail server), it would be blacklisted. There are loads of solutions (services, software and appliances/hardware) that block mail from mailservers listed in these blacklists. Many of these are implemented at a server level (eg by ISPs, company mail servers etc) even though not all client-based solutions have this feature.
