If it's an oligopoly and presumably throwing off cash, what's stopping anyone from creating their own transactional email service?
Answer: It's very difficult. Email is adversarial. Getting to the inbox is incredibly difficult to do _at scale_.
Answer: It's very difficult. Email is adversarial. Getting to the inbox is incredibly difficult to do _at scale_.
A public service where a lot will be spam and unsolicited emails? Must be hell to run.