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Outlook Email Sending Failure – “Message has lines too long for transport”

Kiran Survase 20 Reputation points
2026-05-20T12:16:11.8366667+00:00

Hi,

We are facing an email sending issue from Microsoft Outlook 16.0 when sending emails via SMTP.

Issue Details:

  • Mail Client: Microsoft Outlook 16.0
  • Sender: ******@domain.com
  • Recipient: ******@domains.com
  • Error: message has lines too long for transport (Status: 5.0.0)

Observations:

  • Emails fail specifically when sending replies/forwarded messages.
  • Outlook generates very long MIME/threading headers such as:
    • References
      • Thread-Index
        • X-MS-TNEF-Correlator
        • Content-Type is often multipart/mixed even for simple replies.
        • Plain text and HTML emails also intermittently fail when sent from Outlook.

Impact:

Emails are being rejected by SMTP relay due to malformed or overly long header lines generated by Outlook.

Request:

Kindly assist in confirming:

  • Whether this is a known Outlook 16.0 MIME/TNEF formatting issue
  • Any recommended fixes or patches for Outlook client behavior
  • Best practices to prevent long header generation in reply chainsHi Microsoft Support Team, We are facing an email sending issue from Microsoft Outlook 16.0 when sending emails via SMTP.
Outlook | Windows | Classic Outlook for Windows | For business
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Answer accepted by question author

Michelle-N 16,210 Reputation points Microsoft External Staff Moderator
2026-05-20T13:35:04.9+00:00

Hi @Kiran Survase

Based on the detailed technical data provided, I understand that your organization is experiencing intermittent delivery failures when sending replies or forwarded messages from Microsoft Outlook 16.0 via an SMTP relay. The messages are being rejected with the error "message has lines too long for transport (Status: 5.0.0)" because Outlook generates exceptionally long MIME, threading, and TNEF headers (such as References, Thread-Index, and X-MS-TNEF-Correlator) that exceed the maximum line-length limits enforced by your mail transport server.

Regarding the "message has lines too long for transport (Status: 5.0.0)" error, this issue occurs because your SMTP relay enforces strict line-length limits. Outlook automatically appends unique message identifiers to the References header (as well as its own proprietary Thread-Index header) every time you reply or forward an email. The References header expands significantly as the email chain grows longer, and Outlook is programmed to split (fold) this header line at around 1012 characters. However, standard RFC 5322 / SMTP protocols strictly permit a maximum of only 998 characters per header line.

As an immediate workaround, instead of using the reply or forward buttons, I highly encourage you to temporarily use a new email for these communications. Doing this will completely reset the message headers and prevent the line-length transport error from being triggered.

Please note that we are not Microsoft support; this is a user-to-user support forum. Moderators have no backend access and cannot directly intervene in Microsoft products. We provide only technical guidance and best-practice recommendations based on reported issues. Therefore, in this scenario, please contact official Microsoft support through your Microsoft 365 Admin Center so that their specialized engineers can utilize dedicated tools to assist you further.

Thank you for your understand.

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  1. AI answer

    2026-05-20T12:16:29.27+00:00

    This behavior matches a known issue where Outlook’s Rich Text/TNEF formatting causes headers to become too long, leading to SMTP rejections with 5.x errors.

    From the provided information:

    1. Known Outlook/TNEF-related issue
    • In Rich Text formatted messages, Outlook uses TNEF and places attachment and correlation data into headers such as X-MS-TNEF-Correlator.
    • If the attachment or embedded content is large, the header line length becomes too long and the receiving server rejects the message with a 5.x error.
    • This is documented in Exchange Online as:
      • 550 5.0.350 Remote server returned an error -> 550 x-dg-ref header is too long.
      • Cause: the attachment’s BLOB becomes part of the header stream in X-MS-TNEF-Correlator and exceeds allowed line length.
    • The same underlying mechanism (TNEF/Rich Text causing oversized headers) aligns with the “message has lines too long for transport” error seen when relaying via SMTP.
    1. Recommended fixes/mitigations on the Outlook client

    A. Disable TNEF/Rich Text globally in Outlook 16.0

    • TNEF is what Outlook uses for Rich Text format. Disabling TNEF prevents Outlook from embedding large binary data in headers.
    • Registry-based global configuration:
      1. Create a text file named disable-tnef.reg.
      2. Add the following content and save: 
             Windows Registry Editor Version 5.00
     
     [HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences]
     "DisableTNEF"=dword:1
      3. Run disable-tnef.reg and accept the prompt.
      4. Restart Outlook.
    • This disables TNEF for the user and prevents Rich Text/TNEF headers such as X-MS-TNEF-Correlator from carrying large BLOBs.

    B. Force messages to use HTML or Plain Text (per-message)

    • In Outlook when composing or replying:
      1. Open the message (new, reply, or forward).
      2. Go to Format Text tab → Message Format.
      3. Select HTML or Plain Text (both disable TNEF for that message).
    • This is especially important for long reply/forward chains and messages with attachments.

    C. Avoid Outlook Rich Text format in remote domains and contacts (Exchange/EXO admins)

    • In Exchange Online, TNEF can be controlled on remote domains and on mail contacts/mail users.
    • For external recipients, configure remote domains not to use TNEF/Outlook Rich Text to prevent long TNEF-related headers.
    • See “Message format and transmission in Exchange Online” for details on TNEF control on remote domains and objects.
    1. Best practices to prevent long header generation in reply chains
    • Do not use Outlook Rich Text format for internet mail; standardize on HTML or Plain Text.
    • Disable TNEF globally via the DisableTNEF registry value for affected Outlook 16.0 clients.
    • For Exchange/Exchange Online environments, configure remote domains to never use TNEF for external SMTP domains.
    • Encourage users to periodically trim very long reply/forward chains (especially those with repeated attachments) before sending.
    • For scanning devices or automated systems, ensure they send standard MIME (no TNEF) and avoid unnecessary or repetitive attachments in long threads.
    1. Patches/fixes
    • The documented fix path is configuration-based (disabling TNEF/RTF) rather than a specific Outlook 16.0 hotfix in the provided material.
    • Applying the TNEF-disabling registry setting and enforcing HTML/Plain Text is the recommended mitigation to stop Outlook from generating oversized TNEF-related headers that cause 5.x transport errors.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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