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Bandwidth is a major issue
On a large network you cannot STOP spam, merely filter it. So when your email server receives a million or more spam messages a month ( a figure that is not at all unreasonable) it takes up a large portion of your bandwidth. It IS the ISP's obligation to provide it, as you said. But when a large portion of that bandwidth you are getting according to your contract is spam email traffic...well, it sucks. You get x amount of bandwidth, it doesn't matter if it is legitimate, porn surfing or receiving incoming spam. So spam does take up bandwidth on the incoming side.