App Engine Send Email By Gamil API

App Engine and Gmail API are in Google Cloud Platform. It provides free trial and premium fee is required if it exceeds the quotas.

App Engine(Free Package)
Code and Static Storage: 1GB
Data Store: 1GB
Instance Hour: 28 hours per day (Front-end); 8 hours per day (Black-end)

For Detail:
https://cloud.google.com/appengine/docs/quotas

Gmail API (Free Package)
1,000,000,000 per day

For Detail

https://console.developers.google.com/apis

API Tutorial by Google
https://developers.google.com/gmail/api/

Sample Program