How to Fix \"400 Bad Request Failed Precondition\" Error While Sending Emails via Gmail REST API?

Gmail REST API: Resolving "400 Bad Request Failed Precondition" Error

When attempting to send emails using the Gmail REST API with server-to-server authorization, you may encounter an error message stating "400 Bad Request Failed Precondition." This error can be attributed to various factors, which can be resolved by following specific steps.

Troubleshooting the Error

1. Service Account Authorization:

• Verify that you have configured the service account correctly. Ensure that you have obtained the service account's Client ID and email address.
• Delegate domain-wide authority to the service account in your Google Apps domain admin console.
• Grant the service account access to the required Gmail scopes: https://mail.google.com, https://www.googleapis.com/auth/gmail.compose, https://www.googleapis.com/auth/gmail.modify, https://www.googleapis.com/auth/gmail.readonly

2. Google Credential Configuration:

• Use the service account's Client ID and email address to create a GoogleCredential object.
• Ensure that you are setting the service account's private key from a P12 file.
• Specify the desired service account user email address for the credential.

3. Gmail Service Creation:

• Create a Gmail service object based on the HttpTransport, JsonFactory, and GoogleCredential.
• Set the application name for the Gmail service.

Additional Tips:

• Use the Base64.encodeBase64URLSafeString() method to encode your email message payload.
• Verify that the user from your Google Apps domain you are using for the credential is authorized to send emails.

By following these steps, you should be able to resolve the "400 Bad Request Failed Precondition" error and successfully send emails through the Gmail REST API using server-to-server authorization.

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