A network accepting mail from another network is extending the latter a privilege. It is extremely common sense to block networks from which spam or other abuse is detected. If you really would like to argue that such blocking is illegal, the burden of proof is on you.
That is a simplistic argument from the early days the Internet and is often not true anymore. In many cases IPS's have contracts with users to provide certain services. In other countries Internet service is a public utility. Under US law if you block someone and tell them to make their ISP do something to get unblocked that is technically extortion. In many cases it is certainly proper and allowed to block another network but simplistic arguments will get you into trouble. The specific case I was describing where I tried to reply to someone and was probably not illegal until they told me to make my host do something. It is interesting that the network allowed e-mail to go from their network to mine yet was blocked when I responded. Does anyone check blacklists for outbound mail?
I do like to talk to the ISP and ensure that they address those issues before I relax any filters.
Right, you advocate a "I know abuse when I see it" standard where you have the final say and there is no recourse. If anyone complains they must be a spammer or support spamming? I am now on a Comcast Business IP. At what point or at what level is too much abuse via the Comcast network to get all Comcast customers blocked?
And what standard would that be?
The first standard would be privacy laws (In this case EU laws). Next would be compliance with the posted privacy policy. Microsoft and Cisco play all sorts of tricks here. Microsoft tells the US Government they have corporate privacy program monitored by the TRUSTe program. They tell customers that each of their services has different privacy policies and that some are covered by TRUSTe and some are not. They claim their blacklist services is a service not covered by their main privacy policy and not monitored by TRUSTe. Cisco does exactly the same thing with senderbase.org. The next standard is defamation laws that vary from country to country.