Outlook sync problems almost always come from one of four places: a flaky connection, a stuck cached-mode data file, a corrupted profile, or a server-side hiccup on Microsoft's end. The fixes below are ordered from fastest to most disruptive, so work through them in sequence instead of jumping straight to reinstalling Outlook.

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1. Check the obvious things first

Before touching any settings, confirm Outlook isn't just waiting on something simple:

2. Force a manual sync

Outlook's automatic sync sometimes stalls without throwing an error. Forcing it manually often kicks it loose:

3. Clear out the sync backlog

If sync is just slow rather than fully stopped, Outlook may be working through a large backlog, especially after being offline for a while or after a mailbox migration. You can see this happening in the status bar — it'll say something like "Syncing folder, 1,204 of 6,310 items left." Let it run; interrupting it (closing Outlook, sleeping the laptop) just restarts the count.

Tip: if you're on a laptop, plug into power and disable sleep while a large sync runs. Sync over a slow Wi-Fi connection can take hours; the same sync over wired ethernet often finishes in minutes.

4. Rebuild the cached Outlook data file (OST)

This is the fix that resolves most genuinely stuck syncs. The OST file is Outlook's local copy of your mailbox — when it gets corrupted, Outlook can't reconcile local and server state, even though the connection itself is fine.

  1. Close Outlook completely.
  2. Open Control Panel > Mail (Microsoft Outlook), then click Show Profiles > your profile > Properties > Data Files.
  3. Select the account, click Settings, and note the file location under the Advanced tab — it ends in .ost.
  4. Close that dialog, then locate the file in File Explorer and rename it (e.g. add .old to the end). Don't delete it yet, in case something goes wrong.
  5. Reopen Outlook. It will detect the missing OST and rebuild it fresh from the server, which forces a full, clean resync.

This is safe because the OST is just a cache — your real mail lives on the server (Exchange, Microsoft 365, or your IMAP provider). The rebuild can take a while for large mailboxes, but it fixes sync corruption that no amount of restarting will touch.

5. Rebuild the Outlook profile entirely

If rebuilding the OST doesn't help, the corruption may be in the profile itself rather than just the data file. From Control Panel > Mail, create a brand-new profile, add your account to it, and set Outlook to use the new profile by default on next launch. This is more disruptive — you'll lose any local-only customizations like signatures stored outside your account — but it resolves the deeper class of sync bugs that survive an OST rebuild.

6. When it's not Outlook at all

A few less obvious causes worth ruling out if you've made it this far:

If you're on a managed work account: talk to IT before renaming OST files or rebuilding profiles. Some organizations apply policies (retention, DLP, conditional access) that get reapplied automatically, but it's worth a heads-up so they're not troubleshooting blind if something else breaks.

The bottom line

Most "Outlook won't sync" tickets resolve at step 2 or step 4. The OST rebuild in particular fixes the majority of cases that look mysterious — corrupted local cache, not a broken connection. Reserve the full profile rebuild for when everything else has failed.