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Quick Answer

Wait 15 seconds — Outlook may be syncing. If it stays frozen, press Ctrl + Shift + Esc to open Task Manager, select Microsoft Outlook, click End task, then reopen Outlook. If it freezes again, launch in Safe Mode: press Win + R, type outlook.exe /safe, and press Enter.

How to Fix Outlook Not Responding or Freezing ​

Applies to: Microsoft Outlook (Microsoft 365, Outlook 2021, New Outlook for Windows), Windows 10, Windows 11
Article Type: Troubleshooting
Last Updated: 2026-06-17

Summary ​

This article covers what to do when Outlook freezes mid-use, displays "(Not Responding)" in the title bar, or stops reacting to clicks and keyboard input. These steps address the most common causes — stuck background processes, misbehaving add-ins, oversized mailboxes, and corrupted data files — and most take less than two minutes.

Prerequisites ​

  • A working internet connection (verify by opening a web page in your browser).
  • Microsoft Outlook desktop app — Classic or New Outlook for Windows.

Instructions ​

1. Wait 15 Seconds Before Doing Anything ​

When Outlook shows "(Not Responding)," it is often processing a large operation in the background — syncing a folder, indexing email, or downloading attachments. Windows labels any app as "Not Responding" if it has not replied to input for a few seconds, even if it is still working.

  1. Wait at least 15 seconds without clicking anywhere in the Outlook window.
  2. Watch the status bar at the bottom of Outlook. If it shows a progress indicator (e.g., "Updating inbox..." or a folder sync bar), Outlook is still working — let it finish.
  3. If Outlook recovers on its own, the freeze was temporary. If it happens frequently, continue with the steps below to find the root cause.

2. Force-Close and Restart Outlook ​

If Outlook remains frozen after 30 seconds, a clean restart clears any stuck process.

  1. Press Ctrl + Shift + Esc to open Task Manager.
  2. Find Microsoft Outlook in the list. It may show (Not Responding) next to it.
  3. Click it and select End task.
  4. Wait 10 seconds, then reopen Outlook from the Start menu or taskbar.
  5. If Outlook opens normally and the freeze does not recur, the issue was a one-time stuck process. If it freezes again, continue to Step 3.

3. Start Outlook in Safe Mode ​

Safe Mode launches Outlook with all add-ins disabled. If Outlook runs smoothly in Safe Mode, an add-in is causing the freeze.

  1. Close Outlook completely (use Step 2 if it is frozen).
  2. Press Win + R to open the Run dialog.
  3. Type outlook.exe /safe and press Enter.
  4. Outlook opens with "[Safe Mode]" in the title bar.
  5. Use Outlook for a few minutes — switch folders, open emails, run a search.
  6. If the freeze does not happen in Safe Mode, an add-in is the cause. Continue to Step 4.
  7. If the freeze still happens in Safe Mode, the problem is not add-in related. Skip to Step 5.

4. Identify and Disable the Problem Add-In ​

Add-ins from antivirus software, CRM tools, and PDF converters are frequent causes of Outlook freezing.

  1. Open Outlook normally (not in Safe Mode).
  2. Go to File > Options > Add-ins.
  3. At the bottom, next to Manage, select COM Add-ins and click Go.
  4. You will see a list of all installed add-ins with checkboxes.
  5. Uncheck all add-ins and click OK.
  6. Restart Outlook and use it normally for 10–15 minutes.
  7. If the freeze is gone, one of the disabled add-ins was the cause. Re-enable them one at a time, restarting Outlook after each, until the freeze returns — that add-in is the culprit.
  8. Leave the problem add-in disabled. If it is a business-critical tool, raise a ticket naming the add-in and the version shown in File > Options > Add-ins, and ask IT to install a build that works with your Outlook version.

5. Reduce Mailbox Size ​

Large mailboxes — especially those near the 50 GB or 100 GB limit — force Outlook to index and sync massive amounts of data, which causes freezing.

  1. Go to File > Info and look at the Mailbox Settings section. A storage bar shows how full your mailbox is.
  2. If your mailbox is above 80% full, reduce its size:
    • Delete old emails you no longer need (especially those with large attachments).
    • Empty the Deleted Items folder — deleted emails still count toward your quota.
    • Empty the Junk Email folder.
  3. To find the largest emails: go to your inbox, click the Filter icon, and sort by Size (largest first) or search for size:>10MB.
  4. After cleaning up, Outlook may take a few minutes to re-sync. The freezing should improve significantly.

6. Reduce the Cached Mailbox Window ​

Outlook downloads and stores a local copy of your emails for offline access. The more email it caches locally, the larger the local data file — and the more likely Outlook is to freeze.

  1. Go to File > Account Settings > Account Settings.
  2. On the Email tab, select your account and click Change.
  3. Look for the slider labeled Download email for the past or Mail to keep offline.
  4. Reduce the value. If it is set to All, change it to 12 months or 6 months.
  5. Click Next > Done.
  6. Outlook will remove older items from the local cache (they remain accessible on the server and in Outlook on the web). This reduces the data file size and improves performance.

7. Disable RSS Feeds and Unnecessary Features ​

Outlook's built-in RSS reader and other background features consume resources and can cause freezing, especially on older hardware.

  1. Go to File > Options > Advanced.
  2. Scroll down to the RSS Feeds section.
  3. Uncheck Synchronize RSS Feeds to the Common Feed List (CFL) in Windows.
  4. Scroll further to the Other section.
  5. Uncheck Enable logging (troubleshooting) if it is checked.
  6. Under the Display section, consider unchecking Disable hardware graphics acceleration — try both settings and see which one performs better on your machine.
  7. Click OK and restart Outlook.

8. Repair Your Office Installation ​

If Outlook continues to freeze after addressing add-ins and mailbox size, the Outlook program files themselves may be corrupted.

  1. Close Outlook completely.
  2. Press the Start key, type Installed Apps, and press Enter.
  3. Search for Microsoft 365 (or Office).
  4. Click the … (three dots) next to it and select Modify.
  5. Select Quick Repair and click Repair. This takes 5–10 minutes and does not require internet.
  6. Open Outlook and test. If the freezing continues, repeat the process and select Online Repair instead (this takes 15–30 minutes and requires internet, but is more thorough).

9. Create a New Outlook Profile ​

If repair does not help, the Outlook profile or its local data file (OST) may be corrupted beyond repair. Creating a new profile forces Outlook to build a fresh local copy of your mailbox.

  1. Close Outlook completely.
  2. Press Win + R, type control, and press Enter to open Control Panel.
  3. Search for Mail and open Mail (Microsoft Outlook).
  4. Click Show Profiles.
  5. Click Add and name the new profile (e.g., NewProfile).
  6. Follow the prompts to sign in with your work email address.
  7. Select Always use this profile and choose the new profile from the drop-down.
  8. Click OK and open Outlook. It will create a new data file and re-download your mailbox from the server.

10. When to Contact IT ​

The local fixes are exhausted by this point, so the ticket needs what IT cannot see from their side: the exact wording of any Exchange connectivity or certificate error, how long each freeze lasts, and confirmation that it still happens in Safe Mode on a new profile after an Online Repair.

Contact your IT helpdesk if:

  • Outlook freezes in Safe Mode and after a full Online Repair — the issue may be related to your mailbox configuration on the server.
  • Creating a new Outlook profile did not resolve the freezing.
  • Outlook freezes only when opening emails from specific senders or with specific attachment types — this could indicate a security concern.
  • Multiple colleagues are experiencing the same freezing at the same time — this may indicate a server-side or network issue.
  • You receive errors about Exchange connectivity or certificate warnings during the freeze.

Troubleshooting ​

DANGER

If Outlook works without freezing in Safe Mode (outlook.exe /safe), the cause is almost certainly an add-in. Focus on Step 4. If it freezes even in Safe Mode, focus on mailbox size (Step 5), cache reduction (Step 6), and repair (Step 8).

Symptom / ErrorPotential CauseSolution
"(Not Responding)" for 5–10 seconds, then recoversLarge folder sync or indexingReduce cache window (Step 6) and check mailbox size (Step 5).
Freezes every time you open a specific folderCorrupted item in that folderOpen Outlook on the web, move items from that folder, then try again.
Freezes when searchingSearch index rebuildingWait for indexing to complete, or reduce cached mailbox window (Step 6).
Freezes immediately on startupStuck profile or corrupted data fileStart in Safe Mode (Step 3). If Safe Mode works, create a new profile (Step 9).
Freezes only with a specific add-in enabledAdd-in compatibility issueDisable the add-in (Step 4). Report the issue to IT if it is required for your work.
Freezes after a Windows updateOffice compatibility issueRun Online Repair (Step 8).
"Outlook is using a lot of memory" warningOversized local data fileReduce cache window (Step 6) and clean up mailbox (Step 5).