Seeing 400 error while trying to send a mail

[sudershan1903]

I'm trying to send a mail from my App engine application. I've authenticatied to Gmail API using my service account's key file.  The from mail is my service account mail itself

Seeing the below error in line no 68 - 

<HttpError 400 when requesting https://gmail.googleapis.com/gmail/v1/users/me/messages/send?alt=json returned "Precondition check failed.". Details: "[{'message': 'Precondition check failed.', 'domain': 'global', 'reason': 'failedPrecondition'}]">

Would like to understand what the Precondition check failed error means and any comments on how to resolve the issue.