My problem with all the assumptions laid out: You always have *something*. Since you *ALWAYS* have your phone, and if not, you're using your own computer, etc. . . . Ever been somewhere (like a friend's house) and realize "I forgot my phone, and I need to do something, almost anything, online?". When you LITERALLY have nothing with you, but need to authenticate yourself, how do you do that? And my problem with biometric data is that once it's compromised/impersonated, you're done. I can't change my iris scan/fingerprint/whatever. Sometimes you just need to log in to something on someone else's equipment. And let's not even _start_ the discussion of needing to log in as my aging relatives. Some of them get so confused by an SMS message with an authentication message (and it's almost comical how freaked out they get when they only have 10 minutes to enter the "special code" -- comic if it weren't so sad seeing them get so stressed out.). Good luck telling them to hold on to that FIDO key! Easy always wins.

I used to run my mailserver at my house, when I had the ability to have static IPs (kinda a must), and the ability to specify PTR records (also a must, if you actually want to relay mail out). But since I live in a country (the US) that has such crappy internet connectivity, well, I don't much have that option anymore. Oh, and you also need an ISP that's willing to let port 25 in/out.

One thing I'll echo, and add to: Don't just find a How-To or Walkthrough and blindly follow it. You need to at least understand the basics (such as, why a static IP is necessary, why you need a PTR record, and what the hell is "port 25"). Beyond that. . . I would avoid Linux. I would avoid Windows. I would, and did, use OpenBSD. I guess FreeBSD would work well, too, but I don't much know it.

OK, that's my $0.02 on it. When I was finally forced to not run my server at my house, I had to use a hosting provider. And I wasn't about to use Amazon. But, hay, I just don't like using the most common thing. I only moved off Solaris 10 running on Sparc until I had to remove it from my house.