Unable to add an email address from another domain in GAE

Question

I want to send emails from another account in python mail API GAE. I found this https://cloud.google.com/appengine/docs/python/mail/#Python_Sending_mail

So I tried to authorize my gmail id from Google Developer Console > App Engine > Setting > Edit Application Setting

but this is showing

"Unable to add an email address from another domain"

Edit

I have project ProjectA deployed on [email protected] and I want to add emailId [email protected]

Answer 1

Apparently up until 7th January you need an administrator in order to handle email senders, custom domains and certificates. I tried with a super administrator and it worked. This has been confirmed by Google support in a ticket. Hope it helps.