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Troubleshooting Common Post SMTP Plugin Email Errors

50 threads Sep 17, 2025

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If you're using the Post SMTP plugin and receiving error notifications in your inbox, you're not alone. These alerts are designed to inform you when an email fails to send from your WordPress site. While helpful, the error codes and messages can be confusing. This guide will help you decipher some of the most common errors and provide steps to resolve them.

Why Am I Getting These Error Emails?

The Post SMTP plugin includes a failure notification system. When an email from your site (e.g., from a contact form, WooCommerce order, or WordPress core) fails to deliver, the plugin captures the error and sends a detailed report to your site's WordPress admin email address. This is a core feature to keep you informed of delivery issues.

Common Errors and Their Solutions

1. Invalid "To" or "Cc" Address Errors

Error Examples: Invalid “To” e-mail address “[email protected]”, Invalid “Cc” e-mail address “”, Recipient address required

Why it happens: This is often not a problem with the Post SMTP plugin itself, but with the data it is given. A contact form plugin (like Contact Form 7 or AR Forms) or another process on your site is submitting an email with a blank, malformed, or spammy email address.

How to fix it:

  • Identify the source of the email. The error email usually indicates which form or process triggered it.
  • Check the configuration of that form. Look for fields that collect CC addresses and ensure they are not being submitted blank or with invalid data.
  • If the address looks like spam (e.g., [email protected]), it's likely a spam form submission that was correctly blocked. You can implement a CAPTCHA or other spam prevention tool on your forms.
  • As a temporary diagnostic step, you can try disabling email validation within Post SMTP's settings (Navigate to Post SMTP > Settings > Message). However, this is not a long-term solution, as the root cause is the invalid data from the form.

2. API and Authentication Errors (Gmail, SendGrid, Mailgun)

Error Examples: Bad Request(invalid_grant), Status code is 100, Status code is 0, An unexpected error occurred at Mailgun’s servers

Why it happens: These errors are related to your connection with your third-party mailer (e.g., Gmail, SendGrid, Mailgun). An invalid_grant error typically means the OAuth token for Gmail has expired and needs to be reauthenticated. Transient errors like status codes 100 or 0 can be caused by temporary network glitches or server timeouts between your site and the mailer's API.

How to fix it:

  • For Gmail invalid_grant: You will need to reconfigure the Gmail API connection. This typically involves going to the Post SMTP settings, navigating to the Gmail account configuration, and going through the OAuth setup process again.
  • For transient API errors (100, 0, unreachable servers): First, try clicking the "Resend" link in the Post SMTP email log. If it works on the second try, it was likely a temporary hiccup. If these errors persist, check the status page of your mailer (e.g., status.sendgrid.com) to rule out a service outage. Finally, generate a new Diagnostic Test report from Post SMTP (Post SMTP > Dashboard > Diagnostic Test) to help identify any connectivity issues.

3. RFC Compliance and Spam-Related Errors

Error Examples: INVALID_MSGID Message-Id is not valid, according to RFC 2822, ‘From’ header is missing, 550 High probability of spam

Why it happens: These errors mean your emails are not complying with technical standards (RFCs) or are triggering spam filters. A missing "From" header or malformed "Message-ID" header will cause rejections by strict mail servers like Gmail.

How to fix it:

  • Ensure you are using the latest version of the Post SMTP plugin, as historical issues with the Message-ID format have been addressed in updates.
  • The missing "From" header error has been reported after specific updates. If you encounter this, checking the plugin's support forums for any known issues and ensuring you are on a stable version is recommended.
  • For spam probability errors, use a tool like mail-tester.com to analyze your email's spam score and get specific advice on how to improve it.

4. Mailer-Specific Precondition Errors

Error Examples: Precondition check failed, 5.5.2 Syntax error, 5.7.1 Your message has been rejected because you have been detected sending a virus

Why it happens: Your SMTP mailer (e.g., Office 365, Yandex) has rejected the message due to a policy violation, a malformed command, or content it deems dangerous.

How to fix it:

  • Review the content of the email that failed. If it contains links or attachments, ensure they are safe and not commonly associated with spam.
  • Verify that your account credentials and permissions with your mailer are correct. For example, Office 365 may throw a SendAsDeniedException if the account lacks permission to send from the specified "From" address.
  • Contact your mailer's support for clarification on a specific error code like Precondition check failed, as they can provide the most accurate meaning.

How to Get the Best Help

When seeking help from the community, providing specific information will lead to faster solutions. The most valuable data is the Diagnostic Test report from the Post SMTP dashboard. This report provides essential details about your server environment and configuration that are critical for troubleshooting.

Remember, the error notification system is your first alert that something is wrong. By methodically checking the source of the email, your mailer's connection, and your email's content, you can usually identify and resolve the underlying problem.

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