I strugged with this too and couldn't find instructions online. I'm flying back to the US tomorrow with a Delta unverfied passport. During the check in process in the app there is a screen where you enter your passport details and the first time I checked in I entered the passport information manually. This didn't verify the passport. I then started a text conversation with Delta to "uncheck me in" for the flight. They did and I restarted the check in process. During the second check in process, on the line where you enter your passport number there is a camera symbol to the right of that line. I clicked on it and then took a photo of the picture page of my passport and the app accepted the information. The electronic boarding pass changed from "Verify Docs" (original check in) to "Intl Docs OK" (after the second check in) .