"The daily email quota for billing-enabled apps depends on which billing system your app is using. See Increasing your Daily Mail Quota below to learn how to increase the quota in each case."
"When you enable billing, the email quota remains at 100 messages per day. You must explicitly request a higher mail quota. Go to the Quota Details page of the Admin Console and scroll to the Mail section. There you'll find a link to submit a request for additional email quota. After you submit the request you can monitor its status by visiting the same link again. This will display a page that shows the status of the request: whether the request has been granted, is under review, or has been denied. If the request is granted it may be several minutes before the new quota of 20,000 messages per day takes effect. If the request is denied you cannot make another request to increase mail quota until some time has passed.
If your app needs to send more than 20,000 messages per day, you can sign up for a support package which entitles you to ask for a higher quota."
See this page: https://cloud.google.com/appengine/docs/quotas#Mail
"The daily email quota for billing-enabled apps depends on which billing system your app is using. See Increasing your Daily Mail Quota below to learn how to increase the quota in each case."
Once you go to the referenced page (https://cloud.google.com/appengine/docs/quotas#up_mail_quota), you find this:
"When you enable billing, the email quota remains at 100 messages per day. You must explicitly request a higher mail quota. Go to the Quota Details page of the Admin Console and scroll to the Mail section. There you'll find a link to submit a request for additional email quota. After you submit the request you can monitor its status by visiting the same link again. This will display a page that shows the status of the request: whether the request has been granted, is under review, or has been denied. If the request is granted it may be several minutes before the new quota of 20,000 messages per day takes effect. If the request is denied you cannot make another request to increase mail quota until some time has passed.
If your app needs to send more than 20,000 messages per day, you can sign up for a support package which entitles you to ask for a higher quota."