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If you are using The Newsletter Plugin for WordPress and sending through Office 365 (Microsoft Graph or SMTP), you’ve likely noticed that Microsoft often injects or strips specific headers, which can sometimes interfere with tracking or cause “on behalf of” notices.

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1. Why Office 365 Modifies Headers

Microsoft 365 is a “closed” ecosystem designed for corporate security. When you send a newsletter through their servers:

  • The “From” Address: Must match the authenticated account. If you try to send as [email protected] while logged in as [email protected], Microsoft will either reject it or overwrite the header.

  • X-Headers: Many custom headers (like X-Newsletter-Id) are stripped by Office 365 to prevent tracking or spam-looping.

  • Return-Path: This is almost always rewritten to the primary mailbox address, regardless of plugin settings.

2. Technical Specifications for Removal

To ensure high deliverability and clean formatting, you may need to remove or “sanitize” certain headers before the mail leaves your WordPress site.

Common Headers to Remove/Filter:

Header Reason for Removal/Modification
X-PHP-Originating-Script Can trigger spam filters in Outlook; reveals server paths.
List-Unsubscribe Office 365 sometimes breaks the syntax of custom unsubscribe headers.
Sender If this doesn’t match the From field exactly, Outlook displays “Sent on behalf of…”
X-Mailer Identifies the software (The Newsletter Plugin). Removing it can slightly reduce “bot-like” footprints.

3. How to Implementation Header Cleanup

Since The Newsletter Plugin uses the standard wp_mail() function (or its own SMTP engine), you can control headers using the following methods:

A. Using the “Additional Headers” Section

In the Newsletter Plugin settings under “Advanced Settings”:

  • Ensure the Return-Path is set to the same email as your Office 365 login.

  • Disable “Custom Headers” if you notice Office 365 is flagging the emails as “Spoofed.”

B. Code Snippet (PHP)

If you need to programmatically strip headers before they hit the Office 365 API, you can add this to your functions.php:

PHP

add_filter('newsletter_message_headers', 'clean_office365_headers', 10, 2);

function clean_office365_headers($headers, $email) {
    // Removes the X-Mailer header to look more like a personal email
    unset($headers['X-Mailer']);
    
    // Ensure the Sender header isn't causing "On Behalf Of" issues
    unset($headers['Sender']);
    
    return $headers;
}

4. Crucial Recommendations

  • Use the Microsoft Graph API: Instead of standard SMTP (Port 587), use a dedicated “Postman SMTP” or “WP Mail SMTP” plugin with the Graph API integration. It handles headers much better than the legacy SMTP protocol.

  • SPF/DKIM: Ensure your SPF record includes include:spf.protection.outlook.com. If you are sending from WordPress, your server IP might also need to be included, or you risk the “Unverified Sender” warning.

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