WordPress 6.9 (codenamed “Gene”), released on December 2, 2025, brings significant enhancements to core functionality, particularly with new AI integrations. However, this upgrade has exposed several risks concentrated within the plugin ecosystem.
If you have recently upgraded to WordPress 6.9 and are experiencing email delivery failures, multilingual features (WPML/Polylang) breaking, Elementor editing issues, or WooCommerce causing CPU spikes, you are not alone.
Below is a comprehensive guide to diagnosing and fixing these known issues based on recent testing.
Quick Diagnosis & Fix Checklist
If you are in a rush, use this table to identify your problem and find the immediate solution.
| Step | Goal | Action Detail | Key Search Terms |
| 1. Backup Site | Ensure Data Safety | Use UpdraftPlus or your host’s export tool to create a full backup immediately. | WordPress Backup |
| 2. Check Plugins | Fix Conflicts | Disable non-essential plugins and re-enable them one by one. Watch out for WPML and Borlabs Cookie. | WordPress Plugin Conflict |
| 3. Update Core Plugins | Adapt to 6.9 | Ensure WPML is version 4.8.6+; Borlabs Cookie must be 3.3.20+. | WPML 4.8.6 Compatibility |
| 4. Configure SMTP | Fix Email Issues | Install an SMTP Plugin (e.g., WP Mail SMTP), connect to a service like SendGrid or AliCloud, and test. | WordPress SMTP Setup |
| 5. Performance Rollback | Fix CPU Spikes | If CPU usage spikes and plugin updates don’t help, consider rolling back to 6.8.3 temporarily. | WordPress High CPU Fix |
Fixing Email Sending Failures in WordPress 6.9
The Problem: After upgrading to 6.9, emails stop sending, or SMTP plugins fail to deliver messages.
The Technical Analysis:
WordPress 6.9 officially addressed a known issue (Ticket #49687) via changeset regarding the function and undefined Envelope-From headers. Theoretically, this should improve deliverability and DMARC compatibility by allowing PHPMailer to default the sender to .61010wp_mail()wordpress@<your-domain>
However, if issues persist, they are likely due to server configuration restrictions or plugin conflicts that haven’t adapted to this core change.
The Solution: Configure a Dedicated SMTP Service
1. Verify Core Behavior:
If your logs show errors like , you may need to manually match your “From” address with the authenticated user.451 4.4.8 Unroutable email address. Implement custom filters:
add_action('phpmailer_init', function($phpmailer) {
$phpmailer->Sender = $phpmailer->From;
});2. Switch from PHP Mail to SMTP:
The default PHP function is often unreliable due to hosting restrictions.mail()
- Install a Plugin: Use a dedicated plugin like WP Mail SMTP.
- Connect a Service: Link it to a professional provider (AliCloud Direct Mail, SendGrid, Mailgun, etc.).
- Enable Logging: Turn on error logging within the plugin to trace specific delivery failures.
- Test: Send a test email to verify the connection.
💡 Troubleshooting Tips:
- Server Blocks: Check if your hosting panel has disabled email functions or blocked standard ports. Try using Port 587 (TLS).
- Plugin Conflicts: Form plugins (like Formidable Forms) may report “Success” even when sending fails if the underlying connection is unstable.
- Security Policies: If using Office 365 or similar cloud mail, check if security policies require re-authentication.
Add the following SMTP configuration to the theme’s functions.php file, or save it as a .php file in the plugin directory.
<?php
add_action('phpmailer_init', function($phpmailer) {
// Only fix if using SMTP
if ($phpmailer->Mailer === 'smtp') {
// Set Sender to match From address to satisfy Aliyun SMTP requirements
if (empty($phpmailer->Sender) || $phpmailer->Sender !== $phpmailer->From) {
$phpmailer->Sender = $phpmailer->From;
}
}
}, 10002); // Run after (999) and other pluginsResolving Multilingual Functionality Failures (WPML)
The Problem: Multilingual plugins (specifically WPML or Polylang) stop working, cause the site to crash (Critical Error), or break the block editor after the 6.9 update.
The Technical Analysis:
WordPress 6.9 introduced non-backward-compatible changes to JavaScript module handling, Cache Keys, and Gutenberg Block Editor enhancements. These changes caused a “breaking change” for older versions of translation plugins.
The Solution: Immediate Plugin Updates
1. Update WPML Immediately:
The WPML team has confirmed that a change in WordPress 6.9 broke backward compatibility.
- Action: You must upgrade WPML to version 4.8.6 (or higher). This version was released specifically to fix the Block Editor and caching crashes introduced by WP 6.9.
2. Borlabs Cookie Compatibility:
- If you use Borlabs Cookie, you must update to version 3.3.20 before upgrading WordPress. Older 3.x versions are not compatible.
3. Clear Your Caches:
- After updating plugins, use WP Super Cache, LiteSpeed Cache, or your server cache to clear all data (Object Cache + Browser Cache).
- Check your browser console (F12) for lingering JavaScript errors.
💡 Other Considerations:
- Polylang: Verify you are on the latest version, as JS module changes in 6.9 affect older Polylang builds.
- Page Builders: Check Divi and Elementor for updates, as they often rely on the same core APIs that were modified.
Fixing CPU Spikes and WooCommerce Performance Issues
The Problem: After the upgrade, your server experiences high CPU usage, load spikes, or a “messy” shopping cart experience in WooCommerce.
The Technical Analysis:
While WP 6.9 includes backend performance optimizations, incompatible plugins making calls to deprecated APIs can enter infinite loops or process data inefficiently, causing server resource exhaustion.
The Solution: Isolate and Rollback
1. Isolate the Conflict:
- Disable non-essential plugins. Keep only WordPress Core, WooCommerce, and your Theme.
- If CPU usage drops, re-enable plugins one by one to identify the culprit.
- Once identified, update that specific plugin or find an alternative.
2. The “Nuclear” Option: Rollback:
- If you cannot identify the specific conflict or if a critical plugin hasn’t released a patch yet, the simplest solution is to revert to the previous version.
- Rollback to WordPress 6.8.3 (or your last stable version) and wait for the plugin ecosystem to catch up.
How to Upgrade Safely
- Backup First: Never upgrade your live production site without a backup. Ideally, test the upgrade in a Staging Environment first.
- Update Plugins: The majority of issues reported in the last 48 hours are plugin-related, not core failures. keeping your plugins (especially WPML and SMTP tools) up to date is the best defense against crashes.
WordPress 6.9 is a robust update for core stability and AI capabilities. However, the backend changes require strict plugin compatibility. If you are facing issues, the solution almost certainly lies in updating your plugins or configuring external SMTP services.
Further Reading：Fix WordPress Not Sending Emails: Use Postfix SMTP on Your VPS
