[h1d]

It definitely does not defeat the purpose.

You control the way you authenticate users, how to run spam filtering, etc.

I run my own on Linode with postfix/dovecot/rspamd linked to LDAP for auth that gets routed through SES (probably some cents per month) and it's working good.

These days it's not easy to circumvent being flagged as spam running your own node, because you can look like any spammer until you prove yourself you're not one which takes time and usually not the best idea and you'd rather use an external relay who will provide you with a better reputation from day one.

If you stick with your own delivery route and friends tell you, your mail isn't arriving as they get flagged as spam, that's your fault.