One of the best secure e-mails out there is ProtonMail.
They have a free version, but it survives basically based on donations and people who pay for their services (which are expensive).
From their website:
Today, we help our community of millions of users secure their private data online. More than 10,000 supporters have assisted us in this mission by donating to make this project possible. Thanks to your support, we are continuing to develop state of the art email privacy and security technology from our home base of Geneva, Switzerland.
You have a lot of competition here.... And even for protonmail is hard. They are based in Switzerland, what means that all user data is protect by Switzerland privacy laws, which are very strict. it is also open source...
What would be new in the service you are proposing?
Edit: I am shocked, ProtonMail raised $ 500.000 through crowdfunding!
https://protonmail.com/blog/protonmail-raises-a-record-500000-through-crowdfunding-to-disrupt-the-state-of-email-communications/Good luck