Outlook not syncing means email isn’t arriving, sent messages aren’t showing up on other devices, or the calendar is out of date. The cause depends heavily on which type of email account is failing — Microsoft 365 and Exchange accounts behave very differently from Gmail, Yahoo, or other IMAP accounts, and the fixes are genuinely different. For the bigger picture, our Complete Guide to Fixing Windows, Browser, and Software Errors pulls everything together.
Start with the fastest check: send yourself a test email from another account and wait 60 seconds. If it doesn’t arrive in Outlook, is the send/receive indicator in the status bar showing any activity? If Outlook appears connected and is doing send/receive but mail still isn’t arriving, the problem is likely the mail server — check status.microsoft365.com for Microsoft accounts. If Outlook shows errors or the send/receive animation has stopped, continue below.
Identify Your Account Type First
File → Account Settings → Account Settings → check the Type column. The account type determines which fixes apply:
- Microsoft Exchange or Microsoft 365 → this guide’s Exchange section applies
- IMAP → most Gmail, Yahoo, Apple Mail, and corporate non-Exchange accounts
- POP3 → older email setup; syncing works differently by design
The Universal First Fix: Restart Outlook Properly
Outlook maintains a persistent connection to the mail server. When this connection has stalled — due to a network interruption, a server timeout, or after the computer woke from sleep — the connection shows as active but stops syncing. The fix isn’t just closing and reopening the window; it’s ending all Outlook processes.
Task Manager → find every “OUTLOOK.EXE” process → End task on all of them. Wait 20 seconds. Reopen Outlook. On reconnect, Outlook performs a fresh sync and typically delivers any accumulated mail within a minute.
After reconnecting: press F9 (Send/Receive All) to force an immediate manual sync rather than waiting for the next automatic interval.
Fix 1: Microsoft 365 and Exchange — Credential Issues
Modern Exchange authentication uses OAuth tokens that expire. After a password change, a multi-factor authentication configuration change, or an administrator security policy update, Outlook’s token becomes invalid and sync stops. Outlook sometimes shows “Password Incorrect” prompts; other times it silently fails to sync without any visible error.
Sign out and back in: File → Account Settings → Account Settings → select the Exchange account → Remove → close the dialog → File → Add Account → re-add the account with current credentials. The fresh authentication flow establishes a new valid token. For work accounts with MFA, complete the MFA challenge during sign-in to ensure the token has the correct permissions.
If the account won’t re-add: check that the email address is correct and that the account is active. An IT-deactivated account or a licence change that removed Outlook access produces sync failures that user-side fixes can’t resolve.
Fix 2: Repair the OST File
Exchange and Microsoft 365 accounts use an OST file (offline storage) as a local cache of the mailbox. When this file becomes corrupted — after sudden power loss, disk errors, or interrupted sync operations — Outlook can’t sync correctly because it can’t write to or read from the cache reliably.
Run the built-in repair tool, Scanpst.exe:
- Close Outlook completely (Task Manager → end OUTLOOK.EXE)
- Find Scanpst.exe at
C:Program FilesMicrosoft OfficerootOffice16SCANPST.EXE(path varies by Office version) - Open Scanpst → Browse → navigate to the OST file location (usually
%localappdata%MicrosoftOutlook) - Click Start → if errors found, click Repair
For OST files that are too corrupted to repair: close Outlook → navigate to the OST file location → rename the OST file to .ost.old (don’t delete it yet) → reopen Outlook. It recreates a fresh OST file by syncing from the server. For large mailboxes, the initial sync takes time but produces a clean uncorrupted cache.
Fix 3: IMAP Accounts — Server and Authentication Settings
Gmail, Yahoo, and most non-Microsoft email providers use IMAP. Sync failures on IMAP accounts often trace to changed server settings — providers update authentication requirements, change ports, or switch from password to OAuth and Outlook’s stored configuration becomes invalid.
Gmail made a major change requiring OAuth authentication (instead of regular password) for all IMAP access. If Gmail stopped syncing in Outlook: remove the account and re-add it using “Connect Gmail” rather than “Manual setup” — the Connect Gmail path uses OAuth which Google now requires.
For other IMAP accounts: File → Account Settings → double-click the account → More Settings → Advanced tab → check Incoming server port (should be 993 for SSL/TLS) and SSL settings. Compare these with the provider’s current support documentation — providers occasionally update these settings and previously correct configurations stop working.
Fix 4: Outlook Safe Mode Test
Hold Ctrl while clicking the Outlook icon (or run outlook.exe /safe from Run). Safe Mode loads Outlook with no add-ins and minimal customisation. If sync works correctly in Safe Mode, an add-in is interfering with the normal sync process.
After confirming Safe Mode syncs: exit Safe Mode → File → Options → Add-ins → Manage: COM Add-ins → Go → disable all → restart Outlook. Re-enable add-ins one at a time, testing sync after each. The add-in that breaks sync when re-enabled is the culprit — update it or leave it disabled.
Fix 5: Check Work Rules
Outlook rules process incoming email after it arrives at the server and before it appears in your inbox. A broken rule — one that references a folder that was deleted, a contacts list that no longer exists, or an action the rule can’t complete — causes incoming emails to be lost silently (they arrived but the rule failed to file them) or to trigger errors that slow down sync.
Home → Rules → Manage Rules and Alerts → review every rule. Disable all rules → test whether mail appears in the inbox. If it does, one or more rules were intercepting mail incorrectly. Re-enable rules one at a time to find the broken one and fix or delete it.
Fix 6: Outlook Profile Corruption
The Outlook profile stores account credentials, data file locations, and settings. When a profile becomes corrupted — after certain system events or Office updates — sync fails in ways that individual account fixes don’t resolve. Creating a new profile provides a clean starting point:
Control Panel → Mail → Show Profiles → Add → give it a name → add your email account → set “Always use this profile” to the new profile → OK → open Outlook with the new profile. If sync works in the new profile, the original profile was corrupted. After confirming the new profile works, you can delete the old corrupted profile from the Mail control panel.
Microsoft Support and Recovery Assistant
Microsoft’s SaRA (Support and Recovery Assistant) tool diagnoses and often automatically fixes Outlook sync problems — covering credential issues, OST corruption, profile problems, and server connectivity — without manual steps. Download it from microsoft.com/en-us/download/details.aspx?id=10047 → run → select “Outlook” → let it run the diagnostic. SaRA applies fixes automatically when it identifies a known issue and provides a report of what it found.
This tool handles many of the above fixes automatically. Running SaRA before working through fixes manually is often the fastest path, since it takes 10–15 minutes to check multiple potential causes simultaneously.
Our guide on Outlook not opening covers the launch failures that sometimes precede sync issues when the OST file corruption also prevents Outlook from starting — the Scanpst approach applies to both problems. For Microsoft 365 authentication issues affecting Outlook alongside other apps, our Teams authentication guide covers the OAuth token management that’s common across the Microsoft 365 suite. Microsoft’s Outlook troubleshooting documentation covers the Exchange connectivity tests, AutoDiscover troubleshooting, and the Microsoft Remote Connectivity Analyzer tool for diagnosing Exchange server-side issues from outside the corporate network.
Exchange Online Throttling
Microsoft 365 Exchange Online applies throttling limits to prevent any single mailbox from consuming excessive server resources. When a large mailbox is being heavily accessed — mass email rules processing, large calendar syncs, or Outlook repeatedly attempting to sync a large folder — throttling kicks in and slows or temporarily stops sync. The throttling typically resolves itself within 15–60 minutes as the server releases the limits.
Symptoms of throttling: Outlook shows “Trying to connect” in the status bar during business hours, sync lags by hours rather than seconds, or specific operations (searching, opening large folders) are noticeably slow. If sync normalises on its own after an hour and then slows again during the next busy period, throttling is the pattern. Reducing Outlook’s sync scope — disabling “Download Shared Folders” and “Download Public Folder Favorites” in account settings — reduces the total request volume and helps stay within throttling limits.
Outlook Shared Mailboxes and Delegate Access
Shared mailboxes and delegate access add additional complexity to sync. When a shared mailbox or delegated calendar stops syncing, the cause is often a permission change at the Exchange admin level rather than a local Outlook problem — a colleague had their account permissions changed, or the shared mailbox’s access list was updated by IT.
Check permissions: right-click the shared mailbox or calendar in Outlook’s folder pane → Folder Permissions → confirm your account still appears with the correct access level. If your account isn’t listed or shows different permissions than expected, contact IT. From a local fix perspective: removing the shared mailbox from Outlook (Account Settings → open the Exchange account → More Settings → Advanced → Mailboxes to open) and re-adding it refreshes the access token for that specific mailbox, resolving cases where permission changes weren’t correctly propagated to the local Outlook session.
Send/Receive Groups Configuration
Outlook’s Send/Receive Groups control which accounts sync, how often, and what folder types are included. When these groups are misconfigured — an account removed from the group, sync frequency set to manual, or specific folder types unchecked — email appears to stop syncing even though the account itself is working correctly.
Check: Home → Send/Receive → Send/Receive Groups (dropdown) → Define Send/Receive Groups → select the account in question → look at what’s included. The account should appear in at least one group with appropriate folder selections. Also check the “Schedule an automatic send/receive every X minutes” setting — if this is disabled, Outlook only syncs when you press F9 or close Outlook, which explains why mail isn’t arriving in real time.
Cached Exchange Mode and Folder Download Settings
Exchange accounts in Cached Exchange Mode download a local copy of the mailbox. The “Download the past” setting controls how far back Outlook caches — if this is set to “1 month,” emails older than one month don’t appear in the local cache and seem to not sync. This is expected behaviour, not a bug, but it looks like missing mail.
Adjust: Account Settings → double-click the Exchange account → “Use Cached Exchange Mode” checkbox → “Mail to keep offline” dropdown → set to “All” to download the complete mailbox history. Note: downloading everything significantly increases the OST file size and takes time for the initial sync. For large mailboxes with years of email, the initial full sync after changing this setting may take hours.
Network and Firewall Causes
Outlook requires specific ports to remain open for Exchange sync. For Exchange Online (Microsoft 365): TCP 443 is the primary connection port. For on-premises Exchange servers, additional ports may be required (specific MAPI, RPC, or HTTPS ports depending on the server configuration). When corporate firewalls block these ports for a specific device or user, Outlook shows “Disconnected” or “Trying to connect” indefinitely.
Run the Microsoft Remote Connectivity Analyzer (testconnectivity.microsoft.com) — this tests Exchange connectivity from Microsoft’s servers to confirm whether the Exchange server is reachable and correctly configured. For on-premises Exchange issues, the IT team needs to run the equivalent test from within the corporate network to diagnose internal firewall and routing issues. Users on VPN should also check whether Outlook syncs differently with and without the VPN active — some VPN configurations interfere with Exchange MAPI connections specifically.
AutoDiscover and Profile Setup Errors
AutoDiscover is the mechanism Outlook uses to automatically find Exchange server settings. When AutoDiscover is misconfigured at the server level (incorrect DNS records, expired certificates, or server changes), Outlook can be connected but syncing to incorrect endpoints — fetching mail from a different folder path or a different server than intended, or failing to sync specific folders while others work.
Test AutoDiscover: in Outlook, hold Ctrl and right-click the Outlook icon in the system tray → “Test E-mail AutoConfiguration” → enter your email address → Test. The results show whether AutoDiscover is returning correct server information. If AutoDiscover is failing or returning wrong server information, the fix is on the Exchange admin side — the AutoDiscover DNS records or server configuration needs updating. For personal Microsoft 365 accounts, AutoDiscover issues are rare because Microsoft manages the server configuration centrally. Our guide on Outlook Calendar Not Syncing covers an adjacent issue.
