2. It's easy to check the mailing headers and verify the chains. If one of the hops is an open proxy, you know that anything prior to that is bogus, and there are a number of locations which will give you a free ID and a web interface which will automatically do this for you. Open relays get blacklisted in short order. 99+% of the time, the ip address your mail server got it from is the source, and everything before that is a lie.
Open relays get blacklisted in short order. 99+% of the time, the ip address your mail server got it from is the source, and everything before that is a lie. You're confusing open relay and open proxy.
You can fake some of the header info with an open proxy, but you can't with an open relay.
Very little spam is sent through open proxies and those are also tracked in RBLS.