Mail hosting is not that simple anymore. Your understanding of how it works is missing an entire world of complex issues that you need to solve outside of just hosting a mail server with an open SMTP port.

The biggest certainty is that just having an open port for an SMTP server dangling out there means you will 100% be attacked. Not just sometimes, non-stop. So you don’t want to host on a machine with anything else on it, cuz security. So you need a dedicated host for that portion, and a very capable and restrictive intrusion detection system (let’s say crowdsec), which is going to take some amount of resources to run, and stop your machine from toppling over.

Next, you need all your secondary record systems (SPF, DKIM, DMARC) pointing at a defined and unchanging record for your SMTP server, so you’ll need a static IP. If you don’t have that already, you’re kind of SOL.

Next, you’ll need to be running your own peer authentication system, then a spam filtering system (of which none of them work well without massive amounts of sample data, but you can use public lists to help block known bad actors), decent file threat scanning…you see where I’m going with this. It all takes a fair amount of resources, and even more if/when you get bad actors spamming the machine all the time.

Finally, you’ll probably want this machine completely segmented from the rest of your network, which isn’t really complicated, just costs a bit more money.

There’s a reason why mail hosts and forwarding services cost money, and still exist. It takes a large amount of effort to be somewhat secure, or at least to best of your efforts. With the added costs associated with hosting your own mail servers, most people just avoid the hell out of it. I certainly wouldn’t recommend it.