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Troubleshooting Common Post SMTP Plugin Errors After Updates

19 threads Sep 9, 2025

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Updates to the Post SMTP plugin or WordPress core are essential for security and performance, but they can sometimes introduce unexpected issues that disrupt email delivery. Based on community reports, this guide covers the most frequent errors and their solutions to help you get your emails sending again quickly.

Common Post SMTP Issues & Solutions

1. "Could Not Open Socket" Error

Problem: After an update, emails fail with a "Could not open socket" error. This often indicates a connectivity problem between your server and your SMTP provider.

Why it happens: A plugin update might change how connections are made, or a WordPress core update could interact differently with your server's security configuration. In some cases, web hosts may block outgoing SMTP connections.

Solutions:

  • Check Hosting Restrictions: Contact your hosting provider and ask if they are blocking outgoing SMTP connections. Some hosts have a 'SMTP Tweak' or similar setting in cPanel that needs to be disabled.
  • Run Diagnostic Tests: Navigate to Post SMTP > Dashboard and run the Diagnostic Test and Connectivity Test. This will provide crucial information about your server's environment and connection status.
  • Switch Sockets: In the Post SMTP settings, try switching the socket type (e.g., from SSL to TLS or a different port like 587 instead of 465) to see if a different connection method works.
  • Plugin Conflict: Temporarily deactivate other plugins to rule out a conflict. If the issue resolves, reactivate them one by one to identify the culprit.

2. PHP Warnings and "Headers Already Sent" Errors

Problem: After an update, you see PHP warnings referencing files like PostmanInputSanitizer.php and "Cannot modify header information" errors, often preventing you from saving settings.

Why it happens: These are typically PHP notice or warning messages that are being output to the browser, which interferes with WordPress functions. They often point to a minor code issue in a specific version of the plugin.

Solutions:

  • Update PHP: Ensure you are using a supported version of PHP (7.4 or higher). Older versions may not be fully compatible with updated plugin code.
  • Check for Beta Fixes: The Post SMTP team often releases beta versions to address specific bugs reported by the community. Check the support forums for a mentioned beta version that resolves this issue.
  • Temporarily Disable Debugging: If you have WP_DEBUG enabled in your wp-config.php file, setting it to false may suppress the messages and allow you to save settings until a permanent fix is deployed. Remember to revert this after the issue is fixed.

3. Authentication Failures (401 Errors, "Invalid Grant")

Problem: Emails fail with authentication errors like "5.7.8 Error: authentication failed," "Maximum credits exceeded," or "The provided authorization grant is invalid." This is common with Office 365 and other OAuth-based mailers.

Why it happens: The OAuth token that grants the plugin permission to send emails can expire or be revoked. An update might also change how these credentials are managed.

Solutions:

  • Re-grant Permission: The most common fix is to re-authenticate your email account. Go to your Post SMTP settings, find the option to disconnect or re-authenticate, and then go through the OAuth grant process again.
  • Check Service Limits: Errors like "Maximum credits exceeded" are usually related to limits imposed by your email service provider (e.g., Microsoft 365 quotas), not the plugin itself. Review your account's sending limits with your provider.

4. Settings Page or Save Button Not Working

Problem: The settings page does not load, is stuck on a Freemius screen, or the Save Changes button does nothing when clicked.

Why it happens: JavaScript conflicts, validation errors on the settings form, or issues from a previous incomplete setup can cause this.

Solutions:

  • Browser Console: Open your browser's developer tools (F12) and check the Console tab for errors when you click the save button. Errors like "An invalid form control with name='postman_options[envelope_sender]' is not focusable" often indicate a problem with a hidden required field.
  • Valid From Email: Ensure the "From Email" address in your settings is a complete, valid email address (e.g., [email protected]). Using an incomplete format can prevent the form from saving.
  • Reset/Reinstall: As a last resort, try resetting the plugin to its default settings or completely reinstalling it. Be sure to note your current settings first.

General Troubleshooting Steps

  • Diagnostic Data is Key: When seeking help, always provide the full diagnostic report from Post SMTP > Dashboard > Diagnostic Test. This information is critical for understanding your server environment.
  • Check for Conflicts: Temporarily switch to a default WordPress theme (like Twenty Twenty-Four) and disable all other plugins. If the issue resolves, reactivate them one by one to find the conflict.
  • Rollback Plugin Version: If an update causes critical failure, installing a previous stable version of the plugin can be an effective temporary workaround while waiting for an official patch.

Remember, the Post SMTP – WP SMTP Plugin with Email Logs and Mobile App for Failure Notifications – Gmail SMTP, Office 365, Brevo, Mailgun, Amazon SES and more team actively monitors community forums and often provides beta versions to test fixes for reported bugs. Sharing your diagnostic details and the specific error you encounter is the best way to get assistance.

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