I worked on this for a while, at a time and in a market where most of our recipients had @hotmail addresses. I discovered that mass email sending was akin to a "pay-to-win" game.

We had/opted to acquire the services of a company "expert in email deliverability" (Return Path), who somehow provided detailed metrics of how our IPs were scored by MSFT. I always wondered why MSFT didn't provide those scores by themselves, and how a 3rd. party could have access to them.

Re. your comment... slow ramp-up is the only way, with constant monitoring of deliverability and consequent adjusting of recipients (i.e. removing those who do not open or hard-bounce). I did also wonder if paying that company perhaps gave us a headstart when adding new IPs...