>>106588970 (OP)
I pay one € per month for posteo.
And i do this for over ten years now.
The only downside is that it's from Germany, which means that they are under total surveillance. So i obviously do not use it for accounts on platforms where i say based things.
>>106589819
They are postfix servers with dovecot and the web interface is roundcube. You can run all of this yourself. They are indeed OpenSource. Probably one of the only software-as-a-service providers that you can actually for real run yourself. It's just hard to do it, but possible.
In any way, just use any random email provider that is paid, exists for years already, and uses roundcube.
Without paying, it will be shit.