Mail Undeliverable

Joey Doganieri 0 Reputation points
2026-09-17T17:41:40.2566667+00:00

Whenever i attach a PDF to an email and try sending this to a recipient, I am getting a mail undeliverable error and the email is not going out to the recipient. How do i fix this?

Outlook | Web | Outlook on the web for business | Email
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  1. Demi Nguyen 2,165 Reputation points Independent Advisor
    2026-09-17T18:09:03.3933333+00:00

    Hello Joey Doganieri,

    Thank you for sharing the details.

    Could you please provide the specific error code and diagnostic information shown in the Mail Undeliverable message? If possible, please also share a full screenshot of the error after hiding any personal or sensitive information.

    There are several possible reasons why an email containing a PDF attachment may be rejected, such as message size limits, security policies, or restrictions on the recipient’s mail server. The exact error code will help me identify the reason for the rejection more accurately.

    If this is a Microsoft 365 business account provided by your organization, your IT administrator may have configured message size limits for your mailbox. You can ask your administrator to review the relevant settings using the following documentation: Configure message size limits for a mailbox | Microsoft Learn

    As a temporary workaround, you can upload the PDF to OneDrive and send a sharing link instead of attaching the file directly. You may also try compressing the file before sending it. Microsoft provides additional guidance here: Reduce attachment size to send large files with Outlook | Microsoft Support

    Please send me the error code, diagnostic details, and screenshot so I can better understand the issue and provide the appropriate guidance.

    Kind regards,

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  2. AI answer

    2026-09-17T17:41:58.69+00:00

    If the failure happens only when a PDF is attached, the most likely causes are attachment size limits or the recipient server treating the attachment as spam.

    Try these fixes in order:

    1. Check the file size.
      • Outlook.com cannot send a message with an attachment larger than 25 MB.
      • In some failed-delivery scenarios, attachments larger than 10 MB can also prevent Send Again from working.
    2. Send the file as a cloud link instead of an attachment.
      • Save the PDF to OneDrive and send a sharing link instead of attaching the file.
      • This is the recommended workaround for large files and also helps when attachment filtering is causing the rejection.
    3. Review the non-delivery report for the exact reason.
      • Open the returned postmaster or delivery-failure message.
      • Scroll to the section similar to Diagnostic information for administrators.
      • If it mentions the message was blocked or references blacklist or spam-filter terms, the recipient server is rejecting it as spam.
    4. If the message is being blocked as spam:
      • Contact the recipient through another method and ask them to check whether their email provider or admin is blocking the message.
      • The recipient may need to add the sender to their safe senders list.
      • If the block is on the recipient provider side, their support team may need to unblock the account or domain.
    5. If the address was auto-completed, re-enter it manually.
      • In a new message, delete the recipient from the To field.
      • Start typing the address until it appears in Auto-Complete.
      • Select it with the arrow key and delete that Auto-Complete entry.
      • Type the full address again and resend.
    6. If using Outlook.com in a browser is not already being used, try sending from Outlook.com directly.
      • If the error occurred from Outlook or another email app, sending from Outlook.com can help isolate whether the issue is app-specific.

    If the non-delivery report says the recipient mailbox is full, invalid, or unavailable, the PDF is not the root cause. In that case, the recipient address or mailbox status must be corrected first.

    References:

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

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