A SBC like a Raspberry Pi is sufficient to host mail services for a fairly large number of people. Filter out spam with Spamassassin and a greylist, run sieve to organise your incoming mail flow, use specific mail addresses when communicating with commercial and government organisations and you'll end up asking yourself why so many people insist that self-hosting mail is not an option.
Now that you're self-hosting mail you can also self-host XMPP using the same address making it possible for people to reach you through either SMTP (mail) or XMPP (instant messaging/voice/video calling) using that address. This can be hosted on that same SBC without problems.
Source: my own experience self-hosting mail (and more) since the 90's. If it worked on a 486DX2-66 it should work on a quad-core 1.5GHz 64-bit ARM...
Before even considering why someone might not choose to do so, I would like to point out that selfhosting email is not even that hard to do nowadays. I spent a couple hours a few years back manually setting up a stack on a dummy domain just to see if its as hard as developer circles make it out to be. It was not. Furthermore a quick search today nets half a dozen docker containers you can spin up that claim to be one stop solutions for email. If even a fraction of them succeed in what they claim you could self host email with one command and an env file. You could even use the dockerfiles as a template to run the software on metal, its all there.
Even with this newfound knowledge, and as someone who tries to selfhost equivalents to any service I find myself using regularly, I would never attempt to host my own main emails. My bank accounts are linked to my emails, my investment accounts, my insurance, my loans, things that I am not willing to risk compromising my ability to access as the result of some sort of overly prideful sentiment.
Just because someone has the ability and knowledge to host their own email does not mean they should or would even consider it.
