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.

Just posting the random solutions found online(please check if they are useful):

The "Precondition check failed" error in the context of the Gmail API typically means that one or more required parameters or conditions were not met when making the API request. It's a general error that can have multiple causes.

Here are some possible causes of this error and how to resolve them:

1. Invalid email address or format: Check that the "to" email address is valid and in the correct format. It should be in the format " [email protected] ".

2. Invalid or missing message body: Check that the message body is not empty or invalid. It should contain a valid message in plain text or HTML format.

3. Missing required parameters: Check that all required parameters are included in the API request. For example, you need to provide a valid message ID and message object when calling the messages().send() method in the Gmail API.

4. Insufficient permissions: Check that the service account you are using has sufficient permissions to send emails on behalf of your domain. Make sure that the service account is authorized as a "delegated user" and has the necessary permissions to send emails.

Hi @sudershan1903 , you are not wrong. I tried it both in Dart and Python, there's a similar problem in Dart . Using the raw string on Google's Playground works perfectly.

However, just like @ajojose33333344 explained in his last solution suggestion, when you use a service account as a type of credential Access Credentials , you need to grant it domain-wide authority for it to interact with the user Gmail services.

Here are references: Two-Legged OAuth / Three-Legged OAuth , here's a stack overflow Question .

Hope it helps 🙏🏽 .