Gmail SMTP Errors

Error Code: 429 – User Rate Limit Exceeded

This error occurs when the Gmail API blocks email sending because the user has reached the maximum allowed sending limit. Until this limit resets, new emails cannot be sent.

Causes

The Gmail API limits how many emails you can send in a 24-hour period. You will see this Error Code: 429 – User Rate Limit Exceeded when:

• You send too many emails too quickly
• Your daily quota is exceeded
  • Gmail: 500 emails / 24 hours
  • Google Workspace: 2,000 emails / 24 hours
• An automated process sends emails without rate limits

Once the Google Gmail Workspace email limit is reached, Gmail blocks sending until your 24-hour quota resets.

Solution

To resolve the 429 – User Rate Limit Exceeded error:

• Use scheduled and delayed sending
  Combine retrying with spaced-out delivery. Instead of sending all emails at once, schedule them to send gradually over time. This reduces the chances of hitting Gmail’s daily or hourly limits.
  Tip: Post SMTP Pro includes a Quota Scheduling Add-on that automatically controls email flow and prevents quota overload.
• Throttle email sending
  Limit how many emails are sent per minute or hour to stay safely within Gmail’s API limits.
• Follow the “Retry After” Timestamp
  Gmail includes a “Retry After” value in the error response, usually around 100 seconds, as provided by Google. Wait for this exact duration before sending emails again to avoid repeated 429 errors.

Error Code: 403 – Insufficient Authentication Scopes

This error appears when the Gmail API request is made without the proper permissions (scopes). Since the required scope is missing, Gmail blocks the action and prevents the email from being sent.

Causes

This issue happens when the OAuth access token does not include the necessary Gmail scopes.

Common causes include:

• The user did not grant the required permissions during Google login.
• The app requested limited scopes.
• Existing tokens expired or were created before updating scopes.

When this occurs, Gmail returns 403 – insufficientPermissions.

Solution

To fix the Error Code: 403 Insufficient Authentication Scopes:

Re-authorize the user

Request Google login again and ensure the correct OAuth Gmail scopes are selected:
See our step-by-step guide for how to give the permissions while configuring gmail one-click :

Error Code: 400 – Recipient Address Required

This error occurs when an email is sent without a valid recipient address. Gmail cannot process the request if the To field is empty.

Causes

This issue appears when:

• No email address is provided in the To field
• The recipient field is accidentally left blank
• The system attempts to send an email before filling the required fields

When this happens, Gmail returns a 400 – invalidArgument error.

Solution

To fix the 400 – Recipient Address Required error:

• Ensure the To field is filled
  Always include at least one valid recipient email address before sending. Kindly refer to the documentation for configuring gmail one click.
• Validate email fields
  Add checks in your system to prevent sending an email with empty recipient fields.

Error Code: 400 – Precondition Check Failed

This error appears when a required condition for the Gmail API request is not met. It often happens due to account misconfigurations or temporary restrictions on the user’s Gmail account.

Causes

Common reasons for this error include:

• Gmail account settings are blocking certain actions
• Temporary Google restrictions
• Missing or incorrect configuration in the Gmail API setup
• Trying to perform an action that is not allowed in the current account state

When this happens, Gmail returns an error.

Solution

To resolve the 400 – Precondition Check Failed error:

Verify the Google account and project setup

Ensure the Google account is active, verified, and not restricted.

Make sure the Google Cloud project is fully set up with no pending steps (such as missing billing, verification, or incomplete configuration).

Complete all required Google Cloud configuration steps

This error commonly occurs when some required configurations are incomplete.

Confirm that:

• The OAuth consent screen is fully configured.
• All required APIs are enabled.
• Proper OAuth credentials (Client ID/Secret) have been created. Any missing step can trigger the Precondition Check Failed error.

Retry after some time

If the error is due to temporary Google checks or internal validations, waiting and retrying can resolve it. Retry after 1–5 minutes, and if it still persists, try again after 10–30 minutes.

Error Code: 400 – Missing ‘raw’ RFC822 Payload

This error occurs when the Gmail API request does not contain the required raw email content. Gmail requires every message to include a properly formatted RFC822 raw string (headers + body), or be sent via the /upload/* endpoint for larger payloads.

Causes

This error typically happens when:

• The email content is not generated in valid RFC822 raw format
• Required message fields are empty or missing
• The system sends an incomplete or malformed email payload
• Large or raw messages are not sent through the correct /upload/* endpoint

When this occurs, Gmail returns a 400 – invalidArgument error.

Solution

Verify the email content

Ensure the system generates a complete RFC822 message (headers + body). Missing fields, empty content, or malformed formatting will cause this error.

Check your plugin or site settings

Confirm that:

• From Email is filled
• From Name is filled
• To recipient is valid and not empty

Any missing field prevents Gmail from building the raw payload.

Confirm nothing is stripping the email data

Other plugins, filters, or custom code may modify or remove part of the email content.
Temporarily disable customizations to check if something is interfering with the raw message.