Statistically, a random mailserver on a random IP address is overwhelmingly likely to be sending spam. All the large mail providers use Bayesian filtering and are willing to tolerate a modest false-positive rate to ensure a low false-negative rate.

A spam blocklist is not a court of law. As far as the mail providers are concerned, you're guilty until proven innocent. SPF and DKIM are no guarantee that you're not a spammer.