While today’s Internet offers many modes of communication, one of the oldest modes is still one of the most popular: email. For web applications, email is the primary mechanism for representing and validating identity, and managing access to application-specific accounts. Email is how your app reaches out to your users when they are not on your website and signed in.
An App Engine app can send email messages by calling the Mail service API. An app might send email to notify users of system events or the actions of other users (such as to send social networking invitations), confirm user actions (such as to confirm an order), follow up on long-term user actions (such as to send a shipping notice for an order), or send system notifications to administrators. The app can send email on behalf of itself or the app’s administrators. The app can also send email on behalf of the currently signed-in user, during the request handler.
Sending email messages is similar to initiating HTTP requests: the app calls a service by using an API, and the service takes care of making remote connections and managing the appropriate protocols. Unlike the URL Fetch service, the Mail service does not return a response immediately. Instead, messages are enqueued for delivery, and errors are reported via “bounce” email messages to the sender address.
An app can also receive email messages sent to specific addresses. This might allow an app to provide an email interface ...