Outlook not working? Fix classic Outlook that won't open or stops responding

Outlook not working? Find out whether the problem is Microsoft 365 or your PC, then fix classic Outlook for Windows when it will not open, freezes or stops responding.

Who this is for: people whose classic Outlook for Windows will not start, hangs or shows "Not Responding", and the Microsoft 365 administrators who support them.

Work through the sections in order. They start with the quickest checks and end with the ones that take longest. Stop as soon as Outlook works again.

This guide covers classic Outlook for Microsoft 365, Outlook 2024 and Outlook 2021 on Windows. For new Outlook for Windows it covers only a restart. It does not cover Outlook on the web, Outlook for Mac, the Outlook mobile apps, or mail that is missing or not arriving while Outlook itself runs normally.

Before you start

Check which Outlook you have. If the New Outlook toggle at the top of the window is on, you are using new Outlook: see "If you use new Outlook" below. The other steps are for classic Outlook only.
Office 2016 and Office 2019 are no longer supported when they connect to Microsoft 365 services. If you use one of them, the fix is to move to a supported version.
To check Microsoft 365 service health you need an admin account. People with the Service Support admin or Helpdesk admin role can view it.
Back up your Outlook data file before you repair it or remove a profile. You can find it in Outlook under File > Account Settings > Account Settings, on the Data Files tab.

Check Microsoft 365 service health first

If several people have the same problem at the same time, the cause may be a Microsoft 365 incident. In an incident, people might be unable to send and receive email or unable to sign in. Nothing on your PC will fix that, so check this first.

Go to the Microsoft 365 admin centre at admin.microsoft.com and sign in with an admin account.
In the left navigation, go to Health > Service health. You can also select the Service health card on the Home dashboard.
On the Overview tab, find Exchange Online and look at its Health column.
Look at Active issues Microsoft is working on. Select an issue title to see the affected services, the user impact and the latest updates.

An incident means the service or a major function is unavailable. An advisory means some users are affected but the service still works, often with a workaround. If Exchange Online has an incident, wait for Microsoft to restore the service and follow its updates on this page. If your problem is not listed and many people have it, select Report an issue and complete the short form.

If you cannot sign in to the admin centre at all, check the Microsoft 365 service status page at status.cloud.microsoft for known issues that stop sign-in.

Check for a hidden dialog box or a long task

Press Alt+Tab to look for an open dialog box behind other windows. Outlook stops responding while it waits for an answer, for example the first time you select Dismiss All for reminders. Answer the dialog box.
Look at the status bar at the bottom of the Outlook window. Moving or deleting many messages at once, auto-archive, or sending or receiving a very large attachment can make Outlook look frozen.
If a task is running, wait two or three minutes and see whether Outlook recovers.

Install the latest updates

Install the important, recommended and optional Windows updates, then update Office:

Open Outlook. If it will not open, open another Microsoft 365 app such as Word and start a new document.
In classic Outlook select File > Office Account. In Word select File > Account.
Under Product Information, select Update Options > Update Now. If you do not see Update Now, select Enable Updates first.
When the check finishes, the window says "You're up to date!" or "Updates were installed". Close it and start Outlook again.

If you see only an About button and no Update Options, your updates are managed by your organisation, for example through Group Policy. Ask your IT help desk to install them.

Start Outlook in safe mode and turn off add-ins

Safe mode starts Outlook without add-ins. If Outlook works in safe mode, an add-in is the likely cause.

Close Outlook.
Press Windows key + R to open Run. In Open, type outlook.exe /safe and select OK. You can also hold Ctrl while you start Outlook.
If Choose Profile appears, keep the default Outlook profile and select OK. If asked, enter your password and select Accept.
Check the window: safe mode shows a label at the top and an exclamation mark on the Outlook taskbar icon. If Outlook did not start at all, it is probably not an add-in: go to "Create a new Outlook profile".
If Outlook was stuck on "Processing", close it now and open it normally. That alone can fix it. If it works, stop here.
Select File > Options > Add-ins. Next to Manage, make sure COM Add-ins is selected and select Go.
Write down the names of the ticked add-ins, or take a screenshot of the list.
Clear every check box and select OK. Then select File > Exit.

Important Clear the check boxes only. Do not select an add-in and select Remove: that deletes the add-in if you have admin rights.

Now find the add-in that causes the problem:

Start Outlook normally. If it now works, an add-in was the cause. If it still fails, the add-ins are not the cause: turn them back on from your list and go on to the next section.
Select File > Options > Add-ins > Go, tick one add-in and select OK.
Close and restart Outlook.
Repeat for each add-in on your list. When the problem comes back, the last add-in you turned on is the cause. Start Outlook in safe mode again and clear that add-in. Keep checking the rest, because more than one add-in can cause the error.

Repair your Outlook profile

Use this if Outlook opens but has trouble connecting to your email.

Select File > Account Settings > Account Settings.
On the Email tab, select your account and select Repair.
Follow the prompts in the wizard. When it finishes, restart Outlook.

The Repair option is not shown when Outlook 2016 connects to an Exchange account. If Outlook does not open far enough to reach File, create a new profile instead.

Create a new Outlook profile

Your profile holds your account settings and can become corrupted. A new profile shows whether that is the problem, and it does not delete the old one.

Close Outlook.
Open the profile picker: hold Shift while you start Outlook, or press Windows key + R, type Outlook.exe /profiles and press Enter.
In the profile picker, select Options, then tick Prompt for a profile to be used so Outlook asks which profile to open each time it starts.
Select New, type a name for the profile, for example Outlook New, and select OK.
Add your account: choose or type your email address, for example user@contoso.com, and follow the prompts to sign in.
Start Outlook. In Choose Profile, select the new profile in the list and select OK.

If Outlook now works, the old profile was corrupted. To make the new profile open every time, open the profile picker, select Options, select the new profile and tick Set as default profile.

Note Do not use the Mail icon in Control Panel to add a Microsoft 365 account: it cannot add accounts that need modern authentication. Use the profile picker as above.

Important Removing the old profile is permanent and deletes all its offline cached content. Back up its data files first and remove it only once the new profile has worked for a while.

Repair your Outlook data files

Outlook keeps mail, calendar, contacts and tasks in data files. A Microsoft 365 account uses an offline data file (.ost); other accounts and archives use a .pst file. Either can become damaged.

Recreate the .ost file of a Microsoft 365 account

Important Delete only the .ost file of a Microsoft 365 (Exchange) account, whose mail is kept on the server. Never delete a .pst file: its data is stored only on your computer.

In Outlook, select File > Account Settings > Account Settings and open the Data Files tab. Note the folder and name of the .ost file for your account.
Exit Outlook.
In File Explorer, go to that folder and copy the .ost file to another folder as a backup.
Delete the .ost file in its original folder.
Start Outlook. It creates a new .ost file for your account. Check on the Data Files tab that the .ost file is listed again.

Repair a .pst file with the Inbox Repair tool

Exit Outlook and back up the .pst file.
Open the SCANPST.EXE file that installs with Office. In Outlook 2019 and 2016 it is in C:\Program Files (x86)\Microsoft Office\root\Office16. If it is not there, search for SCANPST.EXE in File Explorer.
Select Browse, choose the .pst file and select Start.
If errors are found, select Repair. The tool saves a backup in the same folder; change the name or folder in Enter name of backup file if you want.
When the repair finishes, start Outlook. If problems remain, run the tool again: it can need several runs.

Items the tool could not put back in place appear in a Lost and Found folder. Items that were permanently deleted or damaged beyond repair may not come back.

Repair Office

A repair replaces damaged Office files. It repairs the whole Office suite, not just Outlook.

Close all Microsoft 365 apps.
On Windows 11, right-click Start and select Installed apps. On Windows 10, right-click Start and select Apps and Features.
Find your Microsoft 365 or Office product. On Windows 11 select the ... next to it; then select Modify.
Select Quick Repair and then Repair. It is faster and replaces only damaged files.
Follow the instructions on screen, restart the computer if asked, and start Outlook.
If Outlook still fails, repeat these steps and select Online Repair instead. It takes longer but makes sure everything is fixed.

Reset the navigation pane

If Outlook still shows "Cannot start Microsoft Outlook. Cannot open the Outlook window", reset the navigation pane, the left pane with your folders. This removes all its customisations.

Close Outlook.
Press Windows key + R. In Open, type Outlook.exe /resetnavpane and select OK.
Outlook starts with the default navigation pane. The setting is stored per computer, so do this on every computer where you use Outlook.

If you use new Outlook

Right-click Start and select Task Manager.
Select Microsoft Outlook and select End task.
Check that Outlook is no longer listed, close Task Manager and open Outlook again.

Check that it worked

Outlook opens normally, not in safe mode: there is no safe mode label at the top of the window.
The title bar no longer shows "Not Responding" when you open messages and folders.
The status bar at the bottom shows no error, and new email arrives in your Inbox.

If Outlook still fails after every section, give your IT support the steps you tried and any error message word for word.

When it goes wrong

Problem	Likely cause	What to do
Several people cannot use Outlook at the same time.	A Microsoft 365 incident.	Check Health > Service health as described above and wait for the service to be restored.
Outlook is stuck on "Processing".	Startup has stalled.	Start Outlook with `outlook.exe /safe`, close it, then open it normally.
"Cannot start Microsoft Outlook. Cannot open the Outlook window."	An add-in, a corrupted profile, a damaged data file or the navigation pane settings.	Work through safe mode, a new profile, the data files and the navigation pane reset, in that order.
Outlook does not start even in safe mode.	Probably not an add-in.	Create a new profile, then repair the data files and Office.
Classic Outlook crashes and offers to start in safe mode.	An older Outlook build with a newer Microsoft Teams Meeting add-in.	Update Office from Word with File > Account > Update Options > Update Now. If you must stay on the old build, clear Microsoft Teams Meeting Add-in for Microsoft Office in the COM add-ins list.
There is no Update Now, only About.	Updates are managed by your organisation.	Ask your IT help desk to install the updates.
There is no Repair button on the Email tab.	Outlook 2016 connected to an Exchange account.	Create a new profile instead. Outlook 2016 is no longer supported with Microsoft 365 services.
The Mail icon in Control Panel will not add your Microsoft 365 account.	It cannot add accounts that need modern authentication.	Create the profile from the profile picker (hold Shift while starting Outlook).