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UmOutlookAddin.dll is the Microsoft Exchange Add-in component used by classic Outlook for Windows. If it causes a crash, the safest fix is to start Outlook in Safe Mode, disable the Microsoft Exchange Add-in, then repair or update Office. Do not download a replacement DLL from an unofficial site. The procedure below also explains how to clear damaged add-in caches, verify the file, and distinguish classic Outlook from new Outlook, where COM add-ins are not supported.
What is UmOutlookAddin.dll and why can it crash Outlook?
UmOutlookAddin.dll is a COM add-in library installed with Microsoft Office. The Microsoft Exchange Add-in uses it for Exchange-related features including Unified Messaging, e-mail permission rules, and calendar availability. Microsoft event-log examples identify the add-in’s ProgID as UmOutlookAddin.FormRegionAddin and show the file in an Office ADDINS directory.
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A crash blamed on this DLL does not prove that the file itself is defective. Outlook may be exposing a conflict with another COM add-in, a damaged Office installation, stale add-in cache data, an incompatible build, or a problem with the Outlook profile or Exchange configuration. The reliable approach is to isolate the add-in first and then repair the Office installation that owns the file.
Which Outlook version does this fix apply to?
These steps apply to classic Outlook for Windows, the desktop program that supports COM add-ins. Microsoft does not support COM add-ins in new Outlook for Windows. If the new Outlook app is open, use a supported web or unified-manifest add-in, or switch to classic Outlook when your organization still depends on this Exchange component.
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| Outlook client | UmOutlookAddin.dll relevance | Recommended direction |
|---|---|---|
| Classic Outlook for Windows | Supports the Microsoft Exchange COM Add-in | Use Safe Mode, COM Add-ins management, and Office repair |
| New Outlook for Windows | COM add-ins are not a supported add-in model | Use a supported web or unified add-in, or open classic Outlook |
Is UmOutlookAddin.dll safe, and where should it be located?
The legitimate component is part of an Office installation, normally beneath an Office program directory in an ADDINS folder. The exact path varies by Office edition, update channel, 32-bit or 64-bit architecture, and whether Office is installed per-user or system-wide.
A Microsoft Office 2016 file table records umoutlookaddin.dll version 16.0.5404.1000, dated 15 June 2023. In that historical table, the x86 file is 1,083,192 bytes and the x64 file is 1,477,432 bytes. Those values describe that particular Office 2016 update; they are not a universal expected version or size for Microsoft 365 and later Office releases.
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The filename alone cannot authenticate an arbitrary copy. Before treating a file as genuine, check its location, open Properties and inspect the Digital Signatures tab, confirm that it belongs to the installed Office product, and review findings from your organization’s security software. A copy in a temporary, user-download, or unrelated application folder deserves investigation; do not infer a malware verdict from the name alone.
How do I diagnose and fix an UmOutlookAddin.dll crash?
- Confirm the client and capture the symptom. Determine whether the failure occurs in classic Outlook for Windows. Note the exact error text, when the crash occurs (startup, opening a message, calendar use, or another action), the Outlook build, and whether the issue affects one Windows account or several. This information prevents you from applying a COM-add-in fix to new Outlook, which uses a different add-in model.
- Start Outlook in Safe Mode. Close Outlook. Press Win+R, type
Outlook /safe, and press Enter. Open the default profile when prompted. Safe Mode starts Outlook without its add-ins. If the crash disappears, an add-in or add-in interaction is the leading suspect; if it still happens, continue with Office and profile troubleshooting rather than assuming this DLL is the cause. - Disable the Microsoft Exchange Add-in without removing it. In Outlook, choose File > Options > Add-ins. At the bottom, set Manage to COM Add-ins and select Go. Clear the checkbox for Microsoft Exchange Add-in, select OK, and restart Outlook normally. During diagnosis, clear the checkbox instead of pressing Remove; removal makes controlled testing and restoration harder.
- Test the result and isolate conflicts. If Outlook is stable with the Exchange Add-in disabled, open the same message, calendar, or folder that previously triggered the failure. If stability returns, re-enable the other add-ins one at a time, restarting Outlook after each change. The add-in that reproduces the crash is the likely trigger. Leave that add-in disabled while you update or repair Office and obtain a compatible version from its publisher or administrator.
- Run Office Online Repair. Close Office programs. Open Control Panel, select the installed Microsoft 365 or Office entry, choose Change, and select Online Repair. Follow the prompts and allow the process to finish before testing Outlook. Online Repair replaces damaged Office components and is the Microsoft-supported repair route for Outlook crashes associated with unhealthy add-ins.
- Update or reinstall Office through its own installer. If Online Repair does not restore the component, install the latest Office updates available to your edition and organization, then test again. When a reinstall is required, use the Microsoft 365 or Office installation workflow so the DLL, registration data, and related files are installed as a matched set. Do not copy a DLL from another computer and do not use a random DLL-download site; a mismatched file can cause registration errors, crashes, or security exposure.
- Clear the classic Outlook add-in cache. Exit Outlook completely, including any Outlook process still shown in Task Manager. In File Explorer, enter
%LOCALAPPDATA%MicrosoftOffice16.0Wefin the address bar and clear the cache contents. For unified-manifest add-ins, also clear%userprofile%AppDataLocalMicrosoftOutlookHubAppFileCache. Microsoft advises clearing the cache completely rather than deleting one manifest file. Reopen Outlook and allow the add-in data to be downloaded again. - Check the manifest and installation state. Outlook add-in manifests are cached under
%LocalAppData%MicrosoftOffice16.0WEF. If the Exchange add-in is missing, remains disabled, or fails during activation, verify that the expected manifest is present and that Outlook can download it. If an administrator-modified manifest was involved, have the Exchange administrator reinstall or restore it through the Exchange Admin Center instead of editing cached files by hand. - Retest with a clean sequence. Restart Windows after repair or cache clearing, open Outlook normally, and reproduce the original action. Test once with the Exchange Add-in disabled and once with it enabled only after Outlook has remained stable. Record which change altered the behavior; that evidence distinguishes an add-in conflict from a broader Office or profile problem.
What should I do when disabling the add-in does not stop the crash?
If Safe Mode and a disabled Exchange Add-in make no difference, the DLL may only be the module that Windows reports when another Office component fails. Work through these checks in order:
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- Run Online Repair even if the add-in checkbox is already clear; the underlying Office files or registration can still be damaged.
- Install pending Office updates for the same architecture as Windows and Outlook. Do not mix x86 and x64 Office binaries.
- Test another Outlook profile if your organization permits it. A profile-specific failure points away from the DLL itself and toward account or local profile data.
- Check Windows Event Viewer under Windows Logs > Application. Record the faulting application, faulting module, exception code, timestamp, and Office version. A module name by itself is not a diagnosis.
- Temporarily test without other COM add-ins, antivirus mail plug-ins, and meeting integrations that load inside Outlook, following your organization’s change-control rules. Re-enable them individually after testing.
How can I tell whether the Office installation is the problem?
Compare behavior across controlled tests rather than relying on the filename. A crash that occurs only in normal mode implicates an add-in or its interaction. A crash that also occurs in Safe Mode, a missing Microsoft Exchange Add-in entry after repair, or repeated registration errors points more strongly to Office installation, profile, or system-state damage.
| Observed situation | Best next action | Reason |
|---|---|---|
| Outlook fails only when add-ins are enabled | Use Safe Mode, then re-enable add-ins one at a time | Identifies the conflicting COM add-in |
| Microsoft Exchange Add-in is unhealthy or disabled | Manage COM Add-ins, then repair or update Office | Uses supported disable, re-enable, reinstall, and repair paths |
| Add-in remains missing after Office repair | Clear Wef and HubAppFileCache, then verify the manifest | Addresses stale or corrupt cached add-in data |
| User is running new Outlook | Use a supported web or unified add-in, or classic Outlook | COM add-ins are not supported in new Outlook |
Can I disable UmOutlookAddin.dll permanently?
You can leave the Microsoft Exchange Add-in unchecked in classic Outlook if Outlook is stable and your work does not require its Exchange-specific functions. Disabling the add-in is reversible and is preferable to deleting or renaming the DLL. Before making the change permanent, confirm with your administrator which Unified Messaging, permission-rule, or calendar-availability features your organization relies on.
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If the checkbox keeps reappearing, the add-in may be controlled by an Office policy or Exchange administrator. In that case, local changes are unlikely to persist; provide the administrator with your Outlook build, the add-in state, and the Event Viewer details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should I record before escalating the problem?
Prepare a short reproducible report so an administrator or Microsoft support can see exactly what changed:
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- Classic or new Outlook, plus the complete Outlook and Office build numbers.
- 32-bit or 64-bit Office architecture and Windows edition.
- The full path and Properties details for
UmOutlookAddin.dll, including its version and digital-signature status. - The precise action that triggers the failure and whether
Outlook /safechanges it. - Which COM add-ins were enabled, which one was disabled, and whether re-enabling it reproduced the crash.
- Event Viewer application entries, exception codes, timestamps, and any Office repair or cache-clearing results.
Contact the Office administrator or Microsoft support when Online Repair, a current Office build, and cache/manifest checks do not restore the component. Avoid deleting the DLL, editing the registry from an unverified guide, or repeatedly reinstalling unrelated software without preserving the evidence above.
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Frequently Asked Questions
Can I delete UmOutlookAddin.dll if Outlook says it is missing?
No. Deleting or replacing the file manually can leave Office registration and related components inconsistent. Repair or reinstall the Office installation that supplied it, then let Outlook recreate its add-in data.
Why does Outlook open in Safe Mode but crash normally?
Safe Mode suppresses Outlook add-ins, so the contrast indicates that an add-in or an interaction among add-ins is involved. It does not identify the guilty component by itself; controlled one-at-a-time re-enabling provides that evidence.
Should I force my Office installation to use version 16.0.5404.1000?
No. That version is a historical Office 2016 file-table value from 15 June 2023, not a universal target for Microsoft 365 or other Office releases. Office update and repair mechanisms select the matching file for your installed build and architecture.
Can Outlook still handle ordinary email with the Exchange Add-in disabled?
Often yes, but the Exchange-specific features supplied by that add-in may be unavailable, and the exact impact depends on your account and organization. Ask your administrator before leaving it disabled if you use Unified Messaging, permission rules, or calendar-availability features.
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