From what I can tell, there are two main reasons companies like Fastmail don't have these problems. The first is that good email providers spend a lot of time

From what I can tell, there are two main reasons companies like Fastmail don't have these problems. The first is that good email providers spend a lot of time maintaining the reputation of the IP addresses they use and shut down bad actors as quickly as possible. The second is that the more people that use the service, the higher percentage of good quality emails being sent, reducing the effect of the occasional bad actor.