Email guide
Outlook Not Syncing With Microsoft 365: A Troubleshooting Guide for Offices
When a user reports that new mail has stopped arriving in Outlook, the mailbox itself is rarely the problem. In most cases the client has lost its connection to Exchange Online, or its local cache has stopped updating. The checks below separate those causes in a sensible order, starting with the ones that affect the whole office.
Establish the scope first
Before changing anything on the user's computer, confirm how far the problem reaches. The answer determines where to look.
- Is Outlook on the web working for the same user? If new mail appears at outlook.office.com but not in the desktop client, the mailbox is healthy and the issue is local to that computer or profile.
- Are other users affected? Several users losing sync at once points to a service incident, a network change, or a recent update rolled out across the office.
- Is it sending, receiving, or both? Messages stuck in the Outbox and messages failing to arrive often have different causes.
Step-by-step
Check Microsoft 365 service health
In the Microsoft 365 admin center, open Health → Service health and review any active advisories or incidents for Exchange Online. If Microsoft has an open incident affecting Outlook connectivity, record the incident ID, inform staff, and avoid making changes on individual machines until it's resolved.
Confirm the client is online
In classic Outlook, check the status bar at the bottom of the window. Working Offline, Disconnected or Trying to connect confirm a connectivity issue. On the Send / Receive tab, make sure Work Offline is not selected. In the new Outlook, an offline banner appears at the top of the window.
Re-authenticate the account
Repeated password prompts, or a yellow Need password notice in the status bar, indicate the client's sign-in token has expired or been revoked, which is common after a password reset or a change to multi-factor authentication. Have the user sign out of Office from File → Office Account, close all Office applications, and sign back in. If the prompt persists, check the user's sign-in logs in the Microsoft Entra admin center for a blocked or failed sign-in.
Check the mailbox quota
When a mailbox reaches its storage quota, Exchange Online first stops it sending and then stops it receiving. In the Exchange admin center, open the mailbox and review its usage. Archiving older items or enabling the online archive resolves this without deleting data.
Rebuild the offline cache (OST)
In Cached Exchange Mode, Outlook works from a local copy of the mailbox. If that file is damaged, the client can appear connected while showing stale data. Close Outlook, open
%localappdata%\Microsoft\Outlook, and rename the user's.ostfile (for example, add.old). When Outlook reopens it downloads a fresh copy from the server. Large mailboxes can take some time to resync; schedule this outside busy hours where possible.Create a new Outlook profile
If the problem returns after an OST rebuild, the profile itself may be corrupt. In Control Panel, open Mail (Microsoft Outlook) → Show Profiles, add a new profile, sign in with the user's account, and set Outlook to use it. Keep the old profile for a few days in case anything needs to be recovered from it.
Verify Autodiscover
Outlook locates the mailbox through Autodiscover. For a custom domain, the DNS zone should include a CNAME record for
autodiscoverpointing toautodiscover.outlook.com. A missing or outdated record, often left over from a previous email provider, causes intermittent connection failures that are difficult to trace from the client side.
Shared mailboxes and delegated calendars
If only shared mailboxes or delegated calendars are out of date, check whether Download shared folders is enabled in the account's Cached Exchange Mode settings. Turning it off makes Outlook read those folders directly from the server, which is often more reliable for large shared mailboxes.
Confirming the fix
Send a test message from an external account and confirm it appears in the desktop client within a minute or two. Ask the user to send a reply and confirm it leaves the Outbox. Where several users were affected, test from at least two computers before closing the ticket.
When to escalate
Escalate if multiple users lose sync with no service advisory in place, if sign-ins are being blocked by Conditional Access, or if messages are missing from Outlook on the web as well as the desktop client. Those situations call for an administrator to review mail flow and tenant configuration rather than further changes to individual computers.