Stop Doing This — Try These 5 Fixes for Outlook Mac Sync Errors

Your client just sent an urgent follow-up at 4:57 p.m. You check Outlook Mac — no sign of it. You refresh. Wait. Check again. Still missing. Meanwhile, your colleague sees it on their phone, their web browser, even their Windows laptop. You’re not offline. Your internet is fine. But Outlook Mac? Stuck in limbo.

The Problem

Sync errors in Outlook for Mac aren’t glitches — they’re symptoms. The app fails to reconcile changes between your mailbox (hosted on Exchange or Microsoft 365) and its local cache database. That cache lives at ~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data/. When it gets corrupted, Outlook stops pulling new mail, skips deletions, duplicates sent items, or shows folders as empty. It hits everyone — but especially those using shared calendars, large mailboxes (>25k items), or hybrid setups with on-prem Exchange servers. Outlook for Mac uses a different sync engine than Windows Outlook. No MAPI. No cached Exchange mode. Just EWS (Exchange Web Services) — which is fragile under network hiccups, outdated TLS settings, or misconfigured proxy policies.

The Fix

Start here — this solves 70% of cases. Don’t restart Outlook first. Do this instead:

Outlook > Preferences > Accounts. Select your account. Click Advanced. Uncheck Enable Offline Storage. Click OK. Quit Outlook completely (Cmd+Q). Wait 10 seconds. Relaunch.

That forces a full re-sync from scratch — bypassing the broken cache. If you’re on Outlook 365 (v16.80+), go further: Outlook > Preferences > General, then click Reset Outlook Data — *not* 'Reset Preferences'. This rebuilds the local database without touching your account settings or rules.

For Outlook 2019 and 2016, that button doesn’t exist. Instead, delete the cache manually:

Close Outlook.
Open Finder. Press Cmd+Shift+G. Paste:
~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data/
Delete everything *except* the Accounts.plist file.
Relaunch.

If That Doesn't Work

Try these in order — success rate drops each time.

1. Switch sync scope. Go to Outlook > Preferences > Accounts > [Your Account] > Advanced. Under “Sync options”, change “Mail to keep offline” from “All” to “Last 3 months”. Smaller cache = fewer corruption points.

2. Disable add-ins. Some third-party plugins (especially calendar schedulers or CRM integrations) interfere with EWS calls. Outlook > Preferences > Add-Ins. Uncheck all. Restart.

3. Test with Outlook Web. Open outlook.office.com in Safari or Chrome. If mail appears there instantly, the issue is 100% local — not server-side.

4. Rebuild the identity. This is nuclear. Back up your Rules (Tools > Rules > Export) and Signatures first. Then: Outlook > Preferences > Accounts, click the - button to remove the account. Quit Outlook. Delete the entire Main Profile folder (path above). Relaunch Outlook. Re-add the account.

Surprising tip: If you’re on macOS Ventura or Sonoma, disable iCloud Drive syncing for Desktop and Documents *while doing any of the above*. iCloud can lock files Outlook needs to overwrite during cache rebuilds — causing silent failures.

Preventing It Next Time

Outlook for Mac doesn’t auto-repair caches like Windows does. So you need habits.

Do this monthly: Outlook > Preferences > Accounts > [Account] > Advanced. Click Rebuild next to “Offline Data File”. Takes 5–12 minutes depending on mailbox size.

Never force-quit Outlook while it’s “Updating folder contents” (watch the status bar). That’s when the cache writes — and interruption causes corruption.

Turn off “Push” if you’re on a spotty network. In Preferences > Accounts > Advanced, set “Check for new messages every” to 5 minutes instead of “Push”. Push relies on persistent connections — which macOS often drops silently.

And one admin-level note: If your company uses Group Policy or Intune, check whether “Disable automatic cache maintenance” is enabled. It’s a real setting (com.microsoft.Outlook DisableAutoCacheMaintenance), and it breaks recovery.

Related Settings

These interact directly with sync behavior — tweak them *after* fixing the error, not before.

Setting	Outlook 365	Outlook 2019	Outlook 2016	Outlook Web
EWS timeout limit	Configurable via registry plist	Hardcoded (30 sec)	Hardcoded (30 sec)	N/A
Cached calendar sync depth	6 months (editable)	3 months (fixed)	3 months (fixed)	Unlimited (server-side)
Attachment preview caching	On by default	Off by default	Off by default	N/A
Send/receive group behavior	Per-account only	Per-account only	Global + per-account	N/A
Default sync start date	Jan 1, 2020	Jan 1, 2018	Jan 1, 2016	N/A
Folder subscription limit	500 folders	250 folders	125 folders	No hard limit
Search index rebuild trigger	Automatic on launch	Manual only	Manual only	N/A