What's So Bad about e-Mail Forwarding?

westfirst asks: "I run a few small domains on a co-lo server. Many of the customers forward their mail from these domains to their home accounts and a surprisingly large number use Road Runner at home. This weekend, Road Runner started blocking all mail from the co-lo farm. The co-lo manager who runs the block of IP addresses seems to feel that this is 'within Road Runner's rights'. They didn't warn anyone and don't seem to be doing much to get the service going again. One customer tells me that, 'Road Runner doesn't accept forwarded mail. They said they finally caught me.' So what's so bad about forwarded mail? Does Road Runner want everyone to use their email services to get people locked into their accounts? Or is this just a last ditch effort to stop the Spamasaurus devouring the net?" This is confusing to me. If none of the users complained about mail from the co-lo, what right does Road Runner have in blocking legal mail for its users? All e-mail is based on forwarding. You break forwarding, and you break SMTP. It's open-relays that are the problem, not anyone who relays. There is a difference here. This behavior is extremely shady to me. I have no problem with ISPs blocking traffic from a location, but if an ISP has cause to do that, then they should say so. What do you think?