Emails are sending back to back

Prevent another resend by taking classic Outlook offline, removing every stale message from its Outbox, and then testing with add-ins disabled before returning Outlook online.

The evidence points to a client-submitted send rather than an Exchange transport-generated resend: the successful Send, OUTLOOK.EXE, MSExchangeRPC, MAPI headers, and ParentFolder: \Outbox identify classic Outlook as the submission path. They do not, by themselves, prove that the user clicked Send; an existing Outbox item or software using Outlook/MAPI can submit it.

Contain the issue and clear pending messages

Perform this on every Windows device or Windows profile where this mailbox is configured:

1. In classic Outlook, select Send/Receive → Work Offline.
2. Confirm the status bar shows Working Offline.
3. Open Outbox and move any required messages to Drafts or delete them.
   • Review all configured accounts and additional Outlook profiles.
   • Preserve a copy outside the Outbox if the message is needed for investigation.
4. Close and reopen Outlook while still offline, then verify that the Outbox remains empty.
5. Select Send/Receive → Work Offline again to reconnect.

A closely matching reported case—old messages resending when Outlook opens—uses this containment sequence before repairing the client state.^1^ Microsoft also documents that messages retained in classic Outlook’s Outbox may be sent later when Outlook becomes able to submit them.^2^

If users switch between new and classic Outlook or send documents through Word/Excel/PowerPoint → Share/Send as Attachment, inspect this workflow specifically. Office’s legacy MAPI handoff can place a message in classic Outlook’s Outbox, where it remains until classic Outlook opens.^3^ To avoid recurrence, create the message directly in the actively used Outlook client and attach the file there instead of using an application’s MAPI-based Send as Attachment command.

Isolate add-ins and MAPI-integrated applications

Since message trace and mailbox auditing are already complete, move to client isolation:

1. Press Windows+R.
2. Run:

      outlook.exe /safe
   

3. Keep Outlook in Safe Mode through a representative test period.
4. If the behavior stops, go to File → Options → Add-ins.
5. At Manage: COM Add-ins, select Go.
6. Disable all add-ins, restart normally, and re-enable them individually until the responsible integration is identified.

Safe Mode starts classic Outlook without add-ins and is Microsoft’s documented method for determining whether an add-in is involved.^4^

Also inventory applications that can create mail through Outlook—document-management tools, CRM software, scanners, PDF tools, reference managers, and Office “send by email” features. Classic Outlook supports the MAPI integrations on which these applications commonly depend.^5^

If the Outbox is empty and Safe Mode does not resolve it

1. Update Windows and Microsoft 365 Apps on the affected workstation. Microsoft recommends installing Office and Windows updates when isolating classic Outlook problems.^4^
2. Create a new Outlook profile and test the mailbox there. Profile repair or replacement is Microsoft’s next supported path for persistent classic Outlook sending problems.^2^
3. If PST files are attached, close Outlook and run the Inbox Repair Tool (SCANPST.EXE) against those files. Do this only after preserving necessary data.
4. Run the Microsoft Classic Outlook troubleshooters; if they do not resolve the behavior, open a Microsoft 365 support case.^6^

There is no Microsoft-published known issue on the current classic Outlook known-issues page that specifically associates build 16.0.20430.20180 with automatically resending months-old messages.^7^

For escalation, retain the affected message’s Internet Message ID, complete audit JSON, resend timestamp, SessionId, client IP, Outlook build/channel, Windows device name, enabled COM add-ins, configured profiles, and whether Outlook had just started. Microsoft identifies ClientIP, ClientInfoString, UserId, and SessionId as key fields for distinguishing sessions during unauthorized-access investigations.^8^

References

1. Outlook resending old emails
2. [Classic Outlook](https://support.microsoft.com/outlook/i-can-t-send-or-receive-messages-in-outlook#make-sure-you're-connected-to-the-internet)
3. New Outlook - Email not sent / delayed when choosing "Share as attachment" from application - Microsoft Q&A
4. [Classic Outlook](https://support.microsoft.com/outlook/getstarted/outlook-for-windows-not-responding-hangs-freezes-or-stops-working)
5. why am I unable to send a document direct out of Word via an email - Microsoft Q&A
6. Issues sending and receiving email | Microsoft Support
7. Fixes or workarounds for recent issues in classic Outlook for Windows ...
8. Learn about properties in Exchange audit records