It is very difficult to run an email provider and not get banned by the others. Google, Microsoft, and Apple control the US market, for example. If they decide your domain is spam, you suddenly can’t email anyone with a Gmail or Hotmail or Apple account. Avoiding getting banned means you have to regulate your own outgoing emails very carefully, rate-limit them just right, and yet also build up a reputation of trustworthiness by sending a lot of emails that don’t get marked as spam.

The only privacy-secure way to do your email would be DIY but this risks getting banned like… all the time.

Personally, I recommend having your own domain and setting up MX records to a reliable email provider that is not one of the big ones and ideally offers some kind of theoretical inbox protection (please note that they could always still read everything if they just copied all incoming messages to another database as well).

Email is itself not very secure. You can use GPG to make it better but most people won’t know how to receive your messages or send secure ones. For security, I recommend using a dedicated e2e chat service or in-person communication.