Email is hard.
Running a mail server with reliable outbound delivery, and inbound spam filtering, is anything but simple.
How is this any harder than anything else in computing?
At work, we handle pretty large traffic for mail, with self-hosted mail servers as well - those are exim. No delivery issues either, although these are only the real, important, not even remotely spam-like, core business messages. Spam-like newsletters are done via sendgrid there.