Don't mix up email clients with email servers.
Thunderbird is an open source email client that I prefer.
Gmail is the email service that I have been using for over 11 years. I wouldn't want to change now as I would have to let everyone* know my new address. *Family, friends, businesses that I deal with...
GMail's servers are owned by Alphabet. Alphabet is a publicly traded company and as a major component of the S&P 500. I follow Warren Buffett's advice and invest in the index, so I suppose I'm a part owner.
I don't know of any privately owned companies that have email servers that the general public can subscribe to. I expect that you would probably have to pay a subscription to use their service.
List of options: https://www.guru99.com/best-free-email-service-providers.html