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Resolving Invalid 'Reply-To' and 'To' Email Address Errors in Post SMTP

15 threads Sep 17, 2025

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If you're using the Post SMTP plugin and encountering errors like Invalid "Reply-To" e-mail address or Invalid "To" e-mail address, you're not alone. This is a common issue that can prevent crucial emails, such as WooCommerce order notifications, from being delivered. This guide will explain why these errors occur and walk you through the most effective solutions.

Why Do These Errors Happen?

These errors are typically not a bug in the Post SMTP plugin itself but are triggered by a specific set of circumstances. The plugin performs validation on email addresses before sending to prevent delivery failures and protect your domain's sender reputation. The error is most often caused by:

  • Incorrectly Formatted Addresses: A user or another plugin submits an email address with a typo (e.g., [email protected] or [email protected]).
  • Plugin Conflicts: Certain plugins, particularly contact form plugins (like Contact Form 7) or abandoned cart plugins (like YITH WooCommerce Recover Abandoned Cart), can sometimes generate malformed email headers that include extra data (e.g., [email protected],Content-Type: text/html).
  • WooCommerce Configuration: WooCommerce often sets the customer's email as the Reply-To address for order notifications. If that customer email is invalid, the entire email send can fail.
  • Server-Side Rules: Some SMTP services or receiving mail servers have strict abuse checks. They may reject emails if the From domain doesn't have proper SPF/DKIM records or if the Reply-To address isn't a valid mailbox on the sending domain.

How to Troubleshoot and Fix the Issue

Solution 1: Override the Reply-To Address (Recommended for WooCommerce)

If the problem is caused by invalid customer emails in WooCommerce, you can force a consistent, valid Reply-To address. This ensures the email is never rejected for this reason.

  1. In your WordPress dashboard, navigate to Post SMTP > Settings.
  2. Click on the Message tab.
  3. Locate the Additional Email Headers section.
  4. Add a new header with the following value: 
    Reply-To: [email protected]
    Replace the email with your preferred address.
  5. Save the changes. This will override any Reply-To header set by other plugins, ensuring it is always valid.

Solution 2: Identify and Resolve Plugin Conflicts

If you see errors where the email address has extra data appended (like ,Content-Type: text/html), the issue is likely a conflict with another plugin.

  1. Identify the Culprit: The error often occurs when sending emails from a specific feature, like a contact form or an abandoned cart test email. Note which plugin is active when the error appears.
  2. Check for Updates: Ensure the conflicting plugin and Post SMTP are both updated to their latest versions.
  3. Contact Support: Reach out to the support team for the other plugin (e.g., YITH, Contact Form 7) and provide them with the exact error message. They may need to patch how their plugin constructs email headers.

Solution 3: Verify Your DNS Records

If your error is related to a 550 bounce message from your SMTP server, your domain's authentication records may be misconfigured.

  1. Ensure you have valid SPF and DKIM records set up for your domain. Your SMTP service provider (e.g., Mailgun, SendGrid) will have documentation on the exact records you need to add to your DNS.
  2. If you are sending from an address like [email protected], make sure that mailbox actually exists or has a forward set up on your mail server. Some strict SMTP servers will verify its existence before accepting the message.

Solution 4: Adjust Fallback Behavior (Advanced)

By default, an invalid Reply-To or To address causes the entire email to fail. If you absolutely need the email to be sent to the admin even when the customer's address is wrong, you would need custom code to catch the error and resend the email without the problematic header. This requires development knowledge.

Conclusion

The most effective and straightforward solution for most users, especially those with WooCommerce stores, is to override the Reply-To address in the Post SMTP settings. This provides a consistent fix that prevents the error from occurring in the first place. For errors involving malformed headers, coordinating with the other plugin's support is the best path forward. Always remember that these validation errors are a protective measure to ensure your important emails are delivered successfully.

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