I want to send emails using python on GAE, so edited \"Email API authorized senders\" in:
https://console.developers.google.com/project/MY_PROJECT/appengine/
The documentation says
All email addresses on the Email API Authorized Senders list need to be valid Gmail or Google-hosted domain Accounts. There are restrictions on who can add whom to the Email API Authorized Senders list:
- Application administrators can add themselves
- Application administrators for applications hosted in a Google Apps domain can add noreply@domain.com, as long as noreply@domain.com is a valid account
- Domain administrators of domains managed by Google Apps can add any user in their domain
So you would need to be a Google Apps admin and the user you're trying to add must also exist in Google Apps. Sadly the documenation doesn't mention groups in the Apps domain which would be quite useful.