Adding this add-ons repository to your Home Assistant instance is pretty easy. Follow the official instructions on the website of Home Assistant, and use the following URL:
https://github.com/fboulay/ha-addons-repository-
Install the ProtonMail bridge to send emails using your ProtonMail account
ProtonMail Bridge is an application available to all paid users that enables the integration of your ProtonMail account with any SMTP clients (in theory). Bridge runs in the background by seamlessly encrypting messages as they leave Home Assistant.
This addon allow to send notification emails using a ProtonMail account.