Emailing is not easy anymore. IP Reputation is the most important thing. They check a lot of stuff with it. Age of domain, how many emails do you send to whom, how many real recipients do you have (if you have a lot of fake email recipients you get banned because of bad email subscription policies), etc etc.
You need accounts on big tech email platforms and check incoming emails from your server, you need postmaster tools and so on. It can be really really hard. A way to make it easier is to use SMTP providers like Mailbaby, SMTP2Go ...
