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They probably use authentication to make sure that you're using their dial-up to access the internet. This was and still is common in the UK, the ISP will allow limited retrieval using a different connection but in order to send you had to be dialing in to their service. The mail server would know that you were not using an IP address assigned by the provider and would block the request.
I've never used netzero so I could be totally wrong here, I'm sure someone will correct me if that is the case.
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