Send & Receive Issues

How to Fix Email Error: Headers Are Too Large

Natalie Taylor Sep 25, 2026 869 views

Question details

The user is experiencing a delivery failure when sending emails to specific recipients, receiving a 'Headers too large' error despite the message containing little text and no attachments.

How to Fix an Email Error Saying Headers Are Too Large

Product: Microsoft Outlook / Email Client
Device & OS: not provided
Scenario: Sending an email message to one or two specific recipients, often as part of an ongoing conversation.
Observed behavior: The email fails to deliver and the server returns a Non-Delivery Report (NDR) stating 'Headers too large'.

Before you start

Ensure you have the exact Non-Delivery Report (NDR) email saved, as it contains specific error codes and routing information that will be vital for your IT administrator to diagnose the issue.

Solution 1Recommended

Send a New Message Without the Conversation Thread

Use this solution to bypass accumulated background metadata and message history that often inflates email header sizes in long reply chains.

Email headers contain invisible routing data. When you reply back and forth in a long thread, especially with recipients using third-party filtering services, the header size grows. Eventually, it exceeds the receiving server's size limit, triggering the 'Headers too large' error.

Create a new email

Open your email client and click on 'New Email' or 'New Message' to start a completely blank draft rather than hitting 'Reply'.

Manually enter the recipient

Type the recipient's email address manually. Avoid using the auto-complete drop-down, as the cached entry might contain corrupted routing data.

Copy essential text only

If you need to reference previous information, copy and paste only the plain text of the essential message body. Do not paste previous email headers, signatures, or complex formatting.

Send the fresh email

Click 'Send' to dispatch the new, clean email and verify if it successfully reaches the recipient without bouncing back.

Send a New Message Without the Conversation Thread

Quick Fix: Starting a fresh email resolves the majority of 'Headers too large' issues caused by excessive message history and repeated filtering stamps.

Solution 2

Provide Error Details to Your Mail Administrator

If sending a fresh email fails, the issue is likely due to server-side routing loops, transport rules, or address expansion requiring administrative intervention.

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Frequently Asked Questions

What does the 'Headers too large' error actually mean?

Every email contains a 'header'—a hidden section of metadata that includes routing information, timestamps, and security filter tags. When an email bounces between servers, especially in long reply chains or through strict spam filters, this metadata grows. If it exceeds the size limit allowed by the receiving server, the email is rejected with this error.

Why does this error only happen with specific recipients?

This usually occurs because the specific recipient's email infrastructure utilizes third-party filtering services (which add extensive data to the headers), has strict transport rules, or uses a complex distribution list (address expansion) that inadvertently inflates the header size.

Can clearing my Outlook auto-complete cache fix this?

Yes, it can. Occasionally, the auto-complete cache stores corrupted or overly complex routing information for a contact. Deleting the recipient's name from your auto-complete list by clicking the 'X' next to their name, and then typing their email address completely fresh, can sometimes resolve hidden header issues.

Outlook & Email Problems