> The problem of identifying if a new IP is used for spam is not easy to solve.

How is it not easy to check a reputation database for domains when evaluating a mail server?

Using DKIM records, Google could cross-reference the reputation for the sending domain and, where applicable, recognize that the domain in question has never been malicious or negligent. And in that case, extend a probationary reputation sufficient to allow the IP to establish its own new reputation.