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Why Your Contact Form Emails Fail When Post SMTP Test Emails Work

38 threads Sep 17, 2025

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The Frustrating Disconnect: Test Emails Pass, Forms Fail

A common and perplexing issue reported by users of 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' plugin is that test emails send successfully, but emails from contact forms (like Contact Form 7, Elementor Forms, or Gravity Forms) fail to deliver or generate errors. This guide will explain why this happens and walk you through the most effective troubleshooting steps.

Why Does This Happen?

The core of the issue lies in the difference between how a test email and a form-generated email are created. The plugin's internal test function creates a simple, standard email. In contrast, a contact form plugin generates a more complex email, often with specific headers, HTML content, and attachments. This difference can expose configuration problems or conflicts that the simple test does not trigger. Common causes include:

  • Incorrect "From" Address Configuration: The address used by the form might not be permitted by your SMTP service (e.g., Gmail, Office 365).
  • Plugin or Theme Conflicts: Another plugin or your theme might be altering the email's headers or content in a way that violates SMTP rules.
  • Attachment Size Limits: File uploads on forms that exceed PHP or server limits can cause silent failures.
  • Improperly Formatted Headers: Contact form plugins sometimes add CC/BCC fields or "From" names in a format that can be misinterpreted.

Step-by-Step Troubleshooting Guide

Step 1: Gather Essential Information

Before making changes, always first check the Post SMTP Email Log. This is your most valuable tool. Look for any failed sends from your form and note the exact error message. Next, navigate to Post SMTP → Dashboard → Troubleshooting → Diagnostic Test. Copy this report, as it contains critical information about your server's configuration.

Step 2: Verify Your "From" Email Address

This is the most common fix. Your SMTP service (Gmail, SendGrid, etc.) requires the "From" address to be an account you own and have permission to send from.

  1. Go to Post SMTP → Settings → Message.
  2. Ensure the "From Email Address" is set to a valid email address on your SMTP service's domain (e.g., [email protected] for Gmail).
  3. Check the box for "Prevent plugins and themes from changing this". This forces all emails, including those from forms, to use this approved address.

Step 3: Check for Conflicts

A conflict with another plugin or your theme is a frequent culprit.

  1. Temporarily switch to a default WordPress theme like Twenty Twenty-Four.
  2. Deactivate all plugins except Post SMTP and your contact form plugin (e.g., Contact Form 7).
  3. Test your form. If it works, reactivate your plugins one-by-one, testing after each, to identify the conflict.

Step 4: Investigate File Uploads

If your form includes file uploads, size matters. A form may upload a file successfully but then fail when trying to attach a large file to an email.

  • Check the file size limit set in your contact form plugin and compare it to your server's PHP settings (upload_max_filesize and post_max_size). The SMTP process may fail if the combined email size is too large.

Step 5: Review Email Headers from the Form

Some users, particularly with Elementor Forms, have reported errors related to empty CC fields or malformed "To" addresses.

  • In your contact form settings, avoid using complex formatting like Name <[email protected]> in the "To" field. Use a plain email address instead.
  • If you are not using CC or BCC, ensure those fields are completely empty in your form's email settings.

Step 6: Confirm OAuth / API Configuration (Gmail/Office 365)

If using Gmail or Office 365 with OAuth, ensure you followed the setup guide correctly. The error "Communication Error [334] – make sure the Envelope From email is the same account used to create the Client ID" indicates a mismatch between your configured email and the OAuth credentials.

When to Seek Further Help

If these steps do not resolve your issue, the problem may be more complex. When seeking help on forums or from your hosting provider, be prepared to provide your Diagnostic Test report and the exact error messages from your Email Log. The more information you can provide, the easier it will be for others to assist you.

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