[google-appengine] Null Pointer exception in Transport.send()

Hi All, I am trying to send some text from Google App Engine to Salesforce but it throws an error "Null pointer exception in Transport.send()". When I debug the message which is being passed in Transport.send() then I see that it is not null. Error is : com.appirio.SendEmail.SendEmailServlet

[google-appengine] The API call mail.Send() was explicitly cancelled.

Hi All, While sending emails from Google app engine to Salesforce, I am receiving "The API call mail.Send() was explicitly cancelled." error message. However, I am using "Task queues" so in next retries this error gets resolved. But I am just curious to know why these errors occur and what if