I think you are making a good point here, that stuff belongs at the interface not the network level and even then it's a tricky problem to solve. We can probably all agree that spam is bad. But can we all agree what is spam? I would bet that we cannot, that people have widely different opinions on what is spam and what isn't because it's not as simple as it sounds. If detecting spam was simple, you wouldn't be told to "check your spam folder" so often, despite the fact that many companies have worked for decades on solving the problem of email spam.