Then I have to buy an IP in a space that has a good reputation. Then I need to setup offsite backups, setup TLS and DKIM, plus a lot of stuff I’m sure I’m missing. Then I have to stay on top of patches and general maintenance. Plus I have to buy a domain name. Suddenly we’re looking at let’s say a 10 year lifespan before you need to upgrade. You are probably going to be basically even on costs but home built has a hundred of hours sunk into it too.
It's not true that you need to spend $200-300 on a server; an email server for home use runs fine on a low-powered fanless Intel Next, for example.
Hardly anyone does offsite backups of their home server, unless they do it by relying on an online backup service. What's the threat-model? Is it that a fire might destroy their server, as well as their in-home backups? OK, backup to a flash key, and give it to a friend to look after.