Code example on how to send mail using the Java low level API. This code generates the appropriate From address based on your application’s ID.
MailService.Message message = new MailService.Message();
message.setSubject(subject);
message.setTextBody(body);
String application_id = SystemProperty.applicationId.get();
String sender = "donotreply@" + application_id + ".appspotmail.com";
message.setSender(sender);
MailServiceFactory.getMailService().sendToAdmins(message);