Where does this extension send data?
This extension exclusively communicates with official Protonmail servers. It does not send any data anywhere else.

What data does it send?
All the requests send the ProtonMail session id and cookies to authenticate with ProtonMails API Servers. Requests are made to the following API endpoints:
- /api/auth/refresh: To refresh the session if it expired.
- /api/messages: To get the inbox emails metadata. (only if "peek" option is enabled).
- /api/messages/count: To update the badge email count. (only if "show unread count on badge" option is enabled).
- /api/messages/read: To mark emails as read.
- /api/settings/mail: To get your ProtonMail "Conversation View" preference.
- /api/events/: Retrieves message events to show notifications on unread email and to update the unread count. (only if notifications enabled)

Other links
You can also read ProtonMail privacy policy on their website: https://protonmail.com/privacy-policy