https://thenextweb.com/apps/2020/09/29/how-jason-fried-wants-to-change-your-perception-of-email-with-hey/

Email is the cornerstone of the current work culture. Even after companies like Slack have tried to eradicate it, email continues to be a large part in the day to day office life.

A few players such as Gmail, Outlook, and Yahoo mail have dominated the email service business for a long time. Then there are some privacy-focused email providers such as Proton Mail and Tutanota with a few million users.