Selfhosted

Sure! That's an SMTP Relay. A lot of folks jumped on the poopoo wagon. It's common wisdom in IT that you don't do your own email. There are good reasons for that, and you should know why that sentiment exists, however; if you're interested in running your own email: try it! Just don't put all of your eggs in one basket. Keep your third party service until you're quite sure you want to move it all in-house (after due diligence is satisfied and you've successfully completed at least a few months of testing and smtp reputation warming).

Email isn't complex. It's tough to get right at scale, a pain in the ass if it breaks, and not running afoul of spam filtering can be a challenge. It rarely makes sense for even a small business to roll their own email solution. For an individual approaching this investigatively it can make sense so long as you're (a.) interested in learning about it, (b.) find the benefits outweigh the risks, and (c.) that the result is worth the ongoing investment (time and labor to set up, secure, update, maintain, etc).

What'll get you in trouble regardless is being dependent on that in-house email but not making your solution robust enough to always fill its role. Say you host at home and your house burns down. How inconvenient is it that your self-hosted services burned with it? Can you recover quickly enough, while dealing with tragedy, that the loss of common utility doesn't make navigating your new reality much more difficult?

That's why it rarely makes sense for businesses. Email has become an essential gateway to other tooling and processes. It facilitates an incredible amount of our professional interactions. How many of your bills and bank statements and other important communication are delivered primarily by email? An unreliable email service is intolerable.

If you're going to do it make sure you're doing it right, respecting your future self's reliance on what present-you builds, and taking it slow while you learn (and document!) how all the pieces fit together. If you can check all of those boxes with a smile then good luck and godspeed says I.