If you are starting a new bulk mail service, you have to be very careful about the IP address. It can't be from a known suspicious IP range (e.g. any part of Amazon EC2). It can't be an IP that already has a bad reputation from a previous service running on it. It can't be an IP that is currently running a bunch of other services and sites.
Even then, you have to "warm up" the IP by slowly increasing the send volume from it, over about a month, while maintaining low spam scores.
It's a pain in the ass, which is why email service provider is a business model.
There are alternatives to email, like targeting and boosting social media posts. But generally speaking, they are all more expensive than email, even taking into account service provider fees.