Microsoft 365 Outlook application opens and then closes immediately

Start classic Outlook in Safe Mode first; if it remains open, disable its COM add-ins because an add-in is the most directly testable cause of a startup crash.

1. Test Outlook without add-ins

1. Press Windows + R.
2. Enter:

Outlook /safe

3. Select OK, choose the default profile if prompted, and sign in.

If Outlook stays open:

1. Go to File → Options → Add-ins.
2. At the bottom, select COM Add-ins → Go.
3. Record or take a screenshot of the enabled add-ins.
4. Clear every selected checkbox, then select OK. Disable them—do not use Remove.
5. Exit and start Outlook normally.
6. Re-enable add-ins one at a time, restarting Outlook after each one. Leave the add-in that causes the crash disabled. ^1^

2. If Outlook also closes in Safe Mode

Run an Online Repair:

1. Close all Office applications.
2. Open Control Panel → Programs → Uninstall a program.
3. Right-click Microsoft 365.
4. Select Change → Online Repair and complete the prompts.
5. Restart Windows and test Outlook. ^2^

3. Check for the rules-related startup crash

A documented Microsoft 365 issue causes Outlook Version 2407 Build 17830.20138 or later to close at startup—even in Safe Mode—when the Application log identifies ucrtbase.dll.

1. Right-click Start → Event Viewer.
2. Open Windows Logs → Application.
3. Find an Event 1000 or 1001 recorded when Outlook closed.
4. Check whether it shows:
   • Faulting application: OUTLOOK.EXE
   • Faulting module: ucrtbase.dll
   • Exception code: 0xc0000005

Only if that signature matches:

1. Close Outlook.
2. Press Windows + R.
3. Run:

Outlook.exe /cleanrules

This removes client and server mailbox rules, so note any important rules first if possible. If Outlook still fails, remove the problematic rules through Outlook on the web. ^3^

4. Test a new Outlook profile

Do not delete the existing profile initially:

1. Search Windows for Control Panel.
2. Select User Accounts → Mail → Show Profiles.
3. Select Add, assign a new profile name, and configure the email account.
4. Start Outlook and select the new profile.

If Outlook opens, the original profile is damaged. Keep the old profile until any locally stored PST files or other data are backed up; removing a profile can remove its associated data files. ^2^

5. Run the classic Outlook troubleshooter

If none of those steps resolves it, run the Classic Outlook Advanced Diagnostics troubleshooter through Microsoft Get Help. It requires:

• Windows 10 or later
• The same PC on which classic Outlook is installed
• Classic Outlook, not new Outlook for Windows

The troubleshooter produces a configuration report and suggests fixes for detected issues. ^4^

References

1. I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
2. Outlook crashes or stops responding when used with Office 365 | Microsoft Support
3. Classic Outlook unexpectedly closes at start up on faulting module ucrtbase.dll | Microsoft Support
4. How to troubleshoot issues that cause Outlook to crash or stop responding when used with Microsoft 365