I think this is relatively common for 'secure e-mail' (e.g. doctor-patient e-mail). You receive an HTTPS link by e-mail, click it, and then receive and enter an

I think this is relatively common for 'secure e-mail' (e.g. doctor-patient e-mail). You receive an HTTPS link by e-mail, click it, and then receive and enter an OTP to read the e-mail content. The difference is that you don't have an account, just individual e-mails. Some sites also use e-mail OTP as a second factor (e.g. Steam and Humble Bundle).