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Fixing Malformed Email Subject Lines and Encoding Issues in Formidable Forms

17 threads Sep 16, 2025

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If you've recently updated your WordPress site or Formidable Forms plugin and started receiving notification emails with garbled subject lines like =?UTF-8?B?V2VsY29tZSB0byBNYWluZSBDb2FzdCBIZXJpdGFnZSBUcnVzdA==?=, you're not alone. This is a common encoding issue that can break your email workflows. This guide will explain why it happens and walk you through the most effective solutions.

Why This Happens: The Double Encoding Problem

This specific issue often arises from a conflict between WordPress core and the Formidable Forms plugin. Historically, a change in WordPress 5.5 introduced a conflict where the plugin's subject line encoding function was being processed a second time by WordPress itself. This double-encoding results in the malformed, unreadable subject lines filled with UTF-8 encoded strings.

How to Fix Malformed Email Subject Lines

Solution 1: Add a Custom Code Snippet (Quick Fix)

The most direct solution is to add a small piece of code that prevents the double encoding. This is a temporary workaround often suggested by the Formidable Forms team while a permanent patch is developed.

  1. Access your WordPress site's files, either via FTP, SFTP, or your hosting provider's file manager.
  2. Navigate to the /wp-content/plugins/formidable/ directory.
  3. Locate and open the formidable.php file for editing.
  4. Scroll to the very bottom of the file and paste the following line on a new line: 
    add_filter( 'frm_encode_subject', '__return_false' );
  5. Save the file and upload it back to your server, overwriting the old one.

Note: This change may be overwritten when you update the Formidable Forms plugin. You may need to reapply it until a confirmed fix is released in a plugin update.

Solution 2: Check the "Prevent plugins and themes from changing this" Setting

As seen in the sample threads, this setting can sometimes affect email formatting. In your Formidable Forms email notification settings, ensure the option "Prevent plugins and themes from changing this" is unchecked. This allows WordPress to handle the encoding correctly.

Solution 3: Investigate Your SMTP and Email Configuration

Since WordPress handles the final email delivery, configuration issues elsewhere can cause problems.

  • Use an SMTP Plugin: If you are not using one, install and configure a dedicated SMTP plugin (such as Post SMTP Mailer/Email Log). This gives you more control over sending and provides valuable logs to see exactly what is being sent from your site.
  • Review Email Logs: Check the logs provided by your SMTP plugin. This will help you confirm whether the malformed subject is leaving your server that way or if it's being altered during delivery by a spam filter.

Other Related Email Issues and Solutions

Encoding and delivery problems can manifest in other ways. Here are quick tips for related issues:

  • HTML Code Showing in Emails: This is typically a conflict with your theme or another plugin altering the email content. The knowledgebase for Formidable Forms has a dedicated article for troubleshooting this.
  • Emails Not Sending or Going to Spam: This is almost always a server configuration issue. Configuring an SMTP plugin to send via a reputable service (like Gmail, SendGrid, or your hosting provider's SMTP) is the most reliable solution.
  • Special Characters Appearing Broken (e.g., š, č, ó): This is a character encoding conflict. Ensure your website's charset is set to UTF-8 and test with the "Send Emails in Plain Text" option both on and off in the Formidable email settings.

By following these steps, you should be able to resolve the malformed subject line issue and ensure your form notifications are delivered cleanly and reliably.

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