What mail server would you recommend to me? The system is a raspi 3B that runs Ubuntu.

I would recommend just plugging mailgun/sendgrid values into postfix, and letting that handle your smtp. It takes the processing load off your machine, and spares you from one of the most difficult, tedious tasks in all of technology: securing an smtp server.

People on here recommended Mailcow to me, so...

You should try this manual:

http://flurdy.com/docs/postfix/

It’s really straight forward. I have my mailserver up and running for a few years now, and I have learned some stuff while setting this up.

@bahua you mean you wouldn't recommend learning sendmail? Those .cf files are basically just random strings

Thanks to everyone. Noted these answers down!
Will check them out! :)