How do I obtain a letter of proof for my services as a reviewer?

This is not something that is generated by OpenReview. Please contact the organizers of the venue where you served as a reviewer to see if they can assist you.

To see a history of all emails sent to you by the organizers, please login and go to openreview.net/messages. To see your recent activity, including any reviews you may have submitted, you can go to openreview.net/activity.

There is also the option of retrieving the public reviews you've written with their corresponding submissions using the API.