MAAWG has published a BCP for vetting new customers. This is the culmination of much work by a lot of people.
One of the best things about the document is the discussion of how spammers attempt to hide their identity. All too often I’ve been called in by ESPs to help them identify how a spammer got on their network and where their process failed. As filtering gets better at blocking spam, spammers are spending more and more time trying to steal good reputations to get their unwanted mail through.
Providers who follow these rules may still find themselves with spammers as customers, but the spammers will have to work harder to get on clean networks.

docs • maawg