I don't think it's difficult, in the sense in which network security, or compiler design, or composing orchestral music are difficult. It can be complicated, be

I don't think it's difficult, in the sense in which network security, or compiler design, or composing orchestral music are difficult. It can be complicated, because there are a lot of moving parts. And because your service is going to deal with many other services that are not under your control, and which you probably never heard of, there are submerged rocks that you can stub your toe on. If you get your ducks lined up first (good domain, clean hosting etc.), then setting up a mailserver can be pretty much a cookie-cutter exercise. There are practical challenges, but no part of setting up a mailserver is technically hard.