When I was selfhosting bulk email, I found it helped to have a large number of good reputation ip addresses, Controls to stop mailing at the first hint of problems, Scrutiny over the email list to avoid sending to honeytrap addresses, overzealous throttling. It can be done it’s just a pain when a mail server gets blacklisted and you have to shift your setup to a new mail server (preferably on a completely different address space to avoid subnet bans).
It’s much easier with todays orchestration systems, but I’m out of that game because unless you are a well known provider some obscure university or small business will block your mailer even if you follow all the rules. Not worth it