Writing applications is hard. Designing a website is hard. Professional work is hard.
But setting up your initial mail server is not hard. Read some guides (ahem: https://flurdy.com/docs/postfix), fix a lot of typos, and you're up an running.
Same with developing the initial version of an application, or designing the first draft. Not rocket science. Not easy but not that hard for an experienced person with simple initial requirements.
But what is hard is keeping it running with more and more users, more requirements and evolving tech and less time to maintain it. Never mind forgotten skills and less priority.
Mail servers have a special knack for running out of disc space or going down for some reason only when you are on holiday and can not get online easily. And you usually only find out after a day or more as you start not receiving messages but don't really notice it for a while.
Or you friends, family or colleagues using it start complaining that some random person they are emailing is not receiving their email and you need to look into it... (Ps. Use DKIM and then DMARC reporting)
If you just want a reliable mail service, just use https://fastmail.com. If you want even more freedom and configurations, do try to set up a mail server. But it is hard. A bit hard.