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Fixing 'Authentication Unsuccessful' and Other Common Post SMTP Errors

55 threads Sep 17, 2025

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Why Am I Getting Authentication and Connection Errors in Post SMTP?

If you use the Post SMTP plugin for WordPress, you've likely encountered a frustrating error message at some point. Based on community support threads, some of the most common issues include authentication failures, connection problems, and configuration errors that prevent emails from sending properly.

Common Post SMTP Error Messages and Their Meanings

1. "5.7.139 Authentication unsuccessful" (Office 365/Microsoft)

This is one of the most frequently reported errors with Office 365/Outlook configurations. The error typically appears as: "5.7.139 Authentication unsuccessful, the request did not meet the criteria to be authenticated successfully. Contact your administrator."

Why this happens:

  • Outdated or incorrect login credentials
  • Microsoft security policies blocking the connection
  • Multi-factor authentication requirements not met
  • Account-specific restrictions from your administrator

Possible solutions:

  • Verify your username and password are correct in Post SMTP settings
  • Check if your Microsoft account requires an app-specific password
  • Ensure your from email matches your Office 365 account email
  • Contact your Office 365 administrator to check for access restrictions

2. "Could not read from [server]" (Mailjet and other providers)

Many users report connection issues with messages like "Could not read from in.mailjet.com" or similar errors with other SMTP providers.

Why this happens:

  • Server connectivity issues
  • Firewall or security restrictions
  • Incorrect port or encryption settings
  • DNS resolution problems

Possible solutions:

  • Run the Post SMTP connectivity test to diagnose connection issues
  • Verify your SMTP server address, port, and encryption settings
  • Check with your hosting provider about firewall restrictions
  • Try alternative authentication methods (Plain vs. Login)

3. API Error Messages (Sendinblue/Brevo, Mailgun, SendGrid)

Various API errors appear with service providers:

  • "Code: 400, Message: Bad Request"
  • "Code: 401, Message: Unauthorized"
  • "Code: 403, Message: Forbidden"
  • "missing_parameter" or "Key not found"

Why this happens:

  • Invalid or revoked API keys
  • Missing required parameters in API calls
  • Unverified sender addresses
  • IP address restrictions

Possible solutions:

  • Regenerate your API key in your email service dashboard
  • Verify your sender email address in your service provider's panel
  • Check if your server IP needs to be whitelisted
  • Ensure you're using the correct API endpoint

4. "Email address is not verified" (Amazon SES)

This error occurs when using Amazon SES: "Message rejected: Email address is not verified."

Why this happens:

  • Using a from address that hasn't been verified in Amazon SES
  • Trying to send from a different region than where your identity is verified

Possible solutions:

  • Verify your email address or domain in Amazon SES console
  • Ensure you're using the same AWS region throughout your configuration
  • Check that your from email matches exactly with your verified address

How to Troubleshoot Post SMTP Issues Effectively

When encountering problems with Post SMTP, follow these systematic troubleshooting steps:

  1. Generate a Diagnostic Report: Navigate to Post SMTP → Troubleshooting Tab → Diagnostic Test. This report provides essential information about your server environment and configuration.
  2. Check Email Logs: Review the Post SMTP email log for specific error messages and patterns. Failed emails will show detailed error information.
  3. Test Connection: Use the built-in connectivity test to verify your server can reach your SMTP provider.
  4. Verify Settings: Double-check all settings including SMTP server, port, encryption type, and authentication method. Sometimes switching between "Plain" and "Login" authentication can resolve issues.
  5. Test with Different Providers: If possible, try configuring a different mailer (Gmail, Office 365, etc.) to isolate whether the issue is provider-specific.
  6. Check for Conflicts: Temporarily disable other plugins to rule out conflicts, especially other SMTP or email-related plugins.

When All Else Fails: Reset and Reconfigure

If you've tried everything and still experience issues, consider resetting Post SMTP completely:

  1. Navigate to the Post SMTP dashboard
  2. Find the option to reset settings (often under Advanced settings)
  3. Run the configuration wizard from scratch
  4. Carefully re-enter all your SMTP credentials and settings

This process often resolves persistent configuration issues that are difficult to pinpoint otherwise.

Conclusion

Most Post SMTP authentication and connection errors stem from incorrect configuration, provider-specific restrictions, or server environment issues. By methodically checking your settings, verifying your credentials, and using the diagnostic tools provided by the plugin, you can usually identify and resolve the problem.

Remember that email delivery involves multiple systems working together—your WordPress site, your SMTP provider, and recipient mail servers. When troubleshooting, consider all these components to effectively diagnose and fix delivery issues.

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