Outlook is not responding, what to do

The Not Responding title bar means Outlook's main thread is busy or stuck, not that the program has necessarily crashed. Short freezes during a big sync or search indexing are normal. Freezes that repeat at the same action point at a small set of causes: a misbehaving add-in, a data file that is damaged or enormous, a leftover Outlook process, or a mailbox operation the server is answering very slowly.

The fix, in order

  1. Give a genuine sync time, once. If the freeze follows opening a huge mailbox or a first sync, wait several minutes and watch the status bar. If Outlook recovers, the cause was workload, not damage, and the later steps are unnecessary.
  2. Kill leftover processes and restart clean. In Task Manager, end every Outlook process, then start Outlook again. A hidden stalled instance from an earlier session makes every new launch look frozen.
  3. Start in safe mode. Run outlook /safe. If Outlook runs smoothly, an add-in is the cause. Disable all COM add-ins under File, Options, Add-ins, then re-enable them in halves to find the one that freezes it.
  4. Repair the data file. Run scanpst.exe against the PST or OST listed in Account Settings, Data Files. For an OST, deleting the file and letting Outlook resync is equally valid and often faster.
  5. Check data file size and mailbox size. A PST approaching its limit, or a mailbox syncing tens of gigabytes in Cached Mode, keeps Outlook permanently busy. Archive old mail, and consider limiting Cached Mode to the last 12 months of mail in Account Settings.
  6. Rebuild the profile, then repair Office. Create a fresh profile in Control Panel, Mail if the freezes follow the account everywhere. If they persist across profiles, run Office Quick Repair, then Online Repair, from Settings, Apps.

If it still happens

Freezes that began immediately after a Windows or Office update are a known pattern; check for a follow-up update first, since Microsoft routinely ships a corrective patch, and use webmail as a stopgap rather than reinstalling everything. An Outlook that freezes only on one specific message or folder is pointing at one corrupt item: move or delete it via webmail.

Common questions

Should I reinstall Outlook to stop the freezing? Almost never as a first move. Reinstalling keeps the same profile, add-ins, and data files, which are where the causes live. Safe mode and the data file repair fix most cases in minutes.

Why does Outlook freeze when I click a particular folder? That folder likely contains a damaged item or the view for it is corrupt. Start Outlook with outlook /cleanviews to reset folder views, and if one folder still freezes it, access the mailbox in webmail and move that folder's contents out in batches.

Related errors

Outlook error 0x80040600 · Outlook search not working · Outlook shows Disconnected