- Software and digital services
- Office suites
Cannot start Microsoft Outlook
Microsoft 365 Cannot start Microsoft Outlook: window fails
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
The message Cannot start Microsoft Outlook (Cannot open the Outlook window) stops classic Outlook from launching. Start Outlook in safe mode with Outlook /safe first to test COM add-ins, and run Outlook.exe /resetnavpane.
Safety first
Stop and check this first
- When disabling COM add-ins, clear the check boxes rather than selecting an add-in and clicking Remove: with administrator rights, Remove can delete the add-in.
- Back up your Outlook data files (.pst and .ost) before removing any profile, because all offline cached content is deleted when you choose Remove.
Safe checks you can perform
- Run the startup troubleshooter or test Outlook /safe
- Reset navigation pane customizations with /resetnavpane
- Create and select a new Outlook profile
Leave these tasks to a qualified professional
- Do not attempt this professional task: Back up data files and run Inbox Repair Tool (scanpst.exe)
- Do not attempt this professional task: Remove trailing space in legacyExchangeDN via ADSIEdit.msc
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Back up data files and run Inbox Repair Tool (scanpst.exe)
What they doWith Outlook exited, a technician backs up your local .pst and .ost data files and runs the Inbox Repair Tool (scanpst.exe) to scan and repair file errors before any broken profile is removed.
They must show youthe backed-up data file path and the completed scanpst.exe repair status.
In writingthe repaired .pst or .ost file path, the active profile name and any COM add-in disabled during isolation.
Remove trailing space in legacyExchangeDN via ADSIEdit.msc
What they doIf the startup error includes The set of folders cannot be opened, a prompt that the Exchange Administrator made a change, or Disconnected status, the administrator opens ADSIEdit.msc, locates the user under CN=Users, and removes the trailing space from the legacyExchangeDN attribute.
They must show youthe legacyExchangeDN attribute in ADSIEdit.msc with no trailing space and Outlook starting after a restart.
In writingthe ADSIEdit.msc attribute change and notice that replies to emails received before the edit may return a non-delivery report (NDR).
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Run the startup troubleshooter or test Outlook /safe
- Who:
- you, on the same Windows 10 or later PC.
- How:
- run the classic Outlook startup troubleshooter in Get Help first; if the error stays, press Windows + R, type Outlook /safe in the Open box, choose OK, and accept the default Outlook profile.
- Result that matters:
- checking whether classic Outlook opens when add-ins are not loaded (safe mode shows a top banner and an exclamation mark on the taskbar icon).
- If Outlook starts in safe mode:
- go to File > Options > Add-ins, choose COM Add-ins > Go, record or screenshot the checked boxes, clear all check boxes (do not click Remove), restart Outlook normally and re-enable add-ins one by one.
- If Outlook still cannot start in safe mode:
- add-ins are not the cause; move on to resetting the navigation pane and testing a new profile.
- Evidence to keep:
- screenshot of the selected COM Add-ins list before clearing the check boxes.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
Reset navigation pane customizations with /resetnavpane
- Who:
- you, after closing classic Outlook.
- How:
- close Outlook completely, press Windows + R to open the Run dialog, type Outlook.exe /resetnavpane in the Open box, and choose OK (run it on each affected computer if you use more than one).
- Result that matters:
- removing local customizations to the leftmost navigation pane to see if they blocked the window from opening.
- If Outlook opens normally:
- the local navigation pane configuration caused the startup failure.
- If the Cannot start Microsoft Outlook message returns:
- test a new Outlook profile next.
- Evidence to keep:
- note of whether Outlook.exe /resetnavpane allowed the window to open.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
Create and select a new Outlook profile
- Who:
- you.
- How:
- create a new Outlook profile, restart Outlook, pick the newly created profile name from the drop-down list in the Choose Profile dialog box, and choose OK.
- Result that matters:
- determining whether your original Outlook profile settings became corrupted.
- If Outlook starts normally with the new profile:
- the old profile is corrupt (back up its data files before removing it).
- If the startup error continues on the new profile:
- repair the data files with scanpst.exe or have IT check the Exchange account.
- Evidence to keep:
- name of the new profile tested and a screenshot of the result at launch.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Software and digital services
- Brand
- Microsoft 365
- Product type
- Office suites
Known code variants
- Cannot open the Outlook Window
- Cannot start Microsoft Outlook
- Cannot start Microsoft Outlook. Cannot open the Outlook window
- No se puede abrir el conjunto de carpetas
- No se puede iniciar Microsoft Outlook
- No se puede iniciar Microsoft Outlook. No se puede abrir la ventana de Outlook
- Outlook /safe
- Outlook.exe /resetnavpane
- scanpst.exe Cannot start Microsoft Outlook
- The set of folders cannot be opened
What it means
In its support guide for Outlook for Microsoft 365, Outlook 2024 and Outlook 2021, Microsoft documents this error when a conflicting COM add-in (such as an outdated ABBYY FineReader add-in), a corrupted Outlook profile, damaged data files (.pst and .ost) or broken navigation pane customizations block startup. On Exchange accounts in Outlook 2016 or 2013, it also appears alongside The set of folders cannot be opened if the legacyExchangeDN attribute ends with a trailing space. Steps can differ on other versions, and the message alone never proves your PC, account or mail server is faulty.
Warnings and stop conditions
- Mediumeditorial warning
When disabling COM add-ins, clear the check boxes rather than selecting an add-in and clicking Remove: with administrator rights, Remove can delete the add-in.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
- Higheditorial warning
Back up your Outlook data files (.pst and .ost) before removing any profile, because all offline cached content is deleted when you choose Remove.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
Symptoms and causes
Probable causes · in order
- Conflicting COM add-in during startupCommon
An add-in loaded into classic Outlook (for example, an outdated version of the ABBYY FineReader add-in) blocks normal launch while allowing Outlook /safe to open.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
- Corrupted navigation pane customizationsPossible
Local customizations in the leftmost pane (folder list and Mail, Calendar, People and Tasks icons) stop the Outlook window from opening until /resetnavpane is run.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
- Corrupted Outlook profile or damaged .pst/.ost data filePossible
When classic Outlook cannot start even in safe mode, the profile holding your settings or the underlying data file needs a new profile test or a scanpst.exe repair.
Evidence: I can't start Microsoft Outlook or receive the error "Cannot start Microsoft Outlook. Cannot open the Outlook Window"
- Trailing space in the Active Directory legacyExchangeDN attributeLess common
On Exchange mailboxes (documented for Outlook 2016 and 2013), a trailing space at the end of legacyExchangeDN triggers this startup error together with an invalid .ost message or Disconnected status.
Evidence: Outlook shows Disconnected in status bar
Virtual technician
Does Cannot start Microsoft Outlook still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Microsoft · 2026 · official_doc
- 3
Outlook shows Disconnected in status bar
Microsoft · 2026 · official_doc
- 4
Outlook muestra Desconectado en la barra de estado
Microsoft · 2026 · official_doc
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