MessageDeliveryFailedException: Could not deliver the message [len=70, data=00000000B7BB764E0DBA30458C8B68C527554FA307008BCEC08141C8A349BBBA1DD4E5B6023600000000010F00008BCEC08141C8A349BBBA1DD4E5B602360001A499EA100000] for PC

Failure code 7d1c has no publicly documented meaning, so first send a simple test from the mailbox provider’s webmail and then troubleshoot either Exchange Online or the third-party mail provider based on where the mailbox is hosted.

1. Isolate New Outlook from the mail service

Identify the provider of the sending account:

• Microsoft 365/Exchange Online: use Outlook on the web.
• Third-party IMAP/SMTP account: use that provider’s webmail site.

Send a new message with one recipient and no attachment or signature.

• Webmail succeeds: the mailbox and basic mail flow work; the problem is likely specific to New Outlook. Proceed to New Outlook diagnostics below.
• Webmail also fails: this is not limited to the Windows client. Follow the matching provider branch below.
• Only one recipient fails: inspect the complete nondelivery report (NDR), verify that address, and ask the recipient or their mail administrator to investigate.

Microsoft’s current New Outlook known-issues page does not list MessageDeliveryFailedException with code 7d1c, so there is no published code-specific workaround at present.^1^

2. Review the complete delivery report

Open the failure message and scroll to Diagnostic information for administrators. The hexadecimal data= value and 7d1c alone do not identify the mail-flow cause. Look for an SMTP response such as:

• 550, 5.1.1, mailbox not found, or user unknown
• domain lookup failure
• blocked or spam-filter rejection
• message too large
• timeout or temporary server failure

Microsoft recommends using that diagnostic section to determine the reason. Also remove the recipient from the To field and Auto-Complete list, type the full address manually, and resend.^2^

If the report says the domain does not exist, verify the address through another channel; that condition must be corrected by the recipient or their domain administrator.^3^

3. If the sender is hosted in Exchange Online

A Microsoft 365 administrator should:

1. Open Microsoft 365 admin center → Health → Service health.
2. Check Exchange Online for an active incident or advisory.^4^
3. Open Exchange admin center → Mail flow → Message trace.
4. Search using the exact sender, recipient, and a date range containing 10/29/2026 10:46:07 PM, accounting for the displayed time zone.
5. Open the result and examine Details and How to fix it.

Message trace identifies whether Exchange Online received, rejected, deferred, or delivered the message and shows the events leading to its final status.^5^ Recent sends can take approximately 5–10 minutes to appear.^6^

Interpret the result from its event details:

• Failed/Rejected: follow the reported SMTP or policy action.
• Delivered: inspect the final event to determine where Exchange delivered or transferred the message; do not rely on the status label alone.
• No result: recheck sender, recipient, time zone, and ingestion delay. A third-party account’s SMTP submission will not appear in Exchange Online message trace.

An administrator can also run Diag: Troubleshoot Email Delivery in the Microsoft 365 admin center; it requires an administrator account and is unavailable in certain sovereign Microsoft 365 environments.^7^

4. If this is a third-party IMAP/SMTP account

Exchange Online message trace does not apply. If the same send fails in the provider’s webmail:

• Give the provider the sender and recipient addresses, timestamp, complete NDR, and failure code.
• Ask the provider to confirm the account is permitted to send and provide its current SMTP authentication requirements.
• Avoid changing ports or encryption settings based on another provider’s configuration.

If webmail succeeds but New Outlook fails, collect New Outlook diagnostics and escalate the client-specific failure.

New Outlook diagnostics

In New Outlook:

1. Select Help → Get Diagnostics.
2. Select Upload Logs or Send Diagnostics and follow the prompts.
3. Use Help → Feedback, include the timestamp and code 7d1c, and permit diagnostic submission if organizational policy allows it.^8^

Do not use classic Outlook profile-repair or .ost deletion instructions for this case; Microsoft states that classic Outlook troubleshooters are not applicable to New Outlook for Windows.^9^

References

1. Fixes or workarounds for recent issues in new Outlook for Windows ...
2. Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com | Microsoft Support
3. Delivery has failed to these recipients or groups - Microsoft Q&A
4. How to check Microsoft 365 service health
5. Trace an email message in Exchange Online
6. Message Trace FAQ in Exchange Online
7. Find and fix email delivery issues as a Microsoft 365 for business admin
8. Contact support and provide feedback in new Outlook for Windows | Microsoft Support
9. Issues sending and receiving email | Microsoft Support