To cut a long story short - you can send emails though the mail service only from emails of accounts (google accounts or other Google app domains) registered as Developers from the Google app engine dashboard (Google App-Engine--> Dashboard--> Developers).
It took me a while to figure this out because:
- It doesn't make any sense: assume you wanna setup an admin account to send out mini-reports from your scheduled cron jobs, in this scenario you're supposed to setup your admin@yourDomain.com as a Developer!?
- I am linking this Google app deployed to appspot to a domain I have registered with Google, so I thought the app could send emails only from accounts registered as administrator of my given domain, which would make slightly more sense!
But then - I can only blame myself 'cause the documentation clearly says:
The sender must be either the address of a registered developer for the application, or the address of the user for the current request signed in with a Google Account.
As a matter of fact, I never read the documentation till I am desperate (who does?).