Well, that's just it, you listed 6 dependencies required to setup a mail server. Then there's the time and effort required to keep everything running.
When it comes to side-projects, mail servers fall into that area where they're distant enough from the thing I'm building to warrant just outsourcing that activity, especially with the number of transactional email providers offering free tiers.
And on top of that, you have a clear separation of responsibility where marketing can do all the A/B testing of the different emails all by themselves, without any effort. It's a great way to reduce bottlenecks in your organisation by separating responsibilities.
When it comes to side-projects, mail servers fall into that area where they're distant enough from the thing I'm building to warrant just outsourcing that activity, especially with the number of transactional email providers offering free tiers.