📧 Recipe · Exchange & Email
Troubleshoot Outlook Not Syncing Email
Diagnose and resolve mail sync failures in Outlook desktop and Outlook on the Web for Microsoft 365 users
Complexity
Intermediate
Impact
end-user-impact + productivity + email + common-ticket
Context
Why This Matters
Outlook sync issues are one of the most common help desk tickets in a Microsoft 365 environment. When a user reports that their inbox is not updating, the cause can range from a transient Exchange Online service issue to a corrupted local OST file, throttled mailbox, blocked device, or mailbox approaching its quota.
This recipe gives you a structured triage path: verify service health, confirm the mailbox is healthy on the server side, check client-side configuration (cached mode, OST, profile), and validate that the user's account and device are not blocked by Conditional Access or MFA prompts. Running these checks in order prevents wasted time on client-side fixes when the root cause is server-side (or vice versa).
When to run this recipe
- User reports Outlook inbox is stale or not receiving new email
- Send/Receive in Outlook desktop hangs or fails silently
- Outlook on the Web (OWA) shows new mail but desktop Outlook does not
- After a password reset, device migration, or network change
Expected Outcomes
After completing this recipe you will have:
- Confirmed whether the issue is tenant-wide (service health), mailbox-specific, or client-specific
- Validated the user's mailbox quota, litigation hold state, and ActiveSync/Outlook connectivity
- Identified any Conditional Access, MFA, or token-revocation events blocking the client
- Applied the appropriate fix: OST rebuild, profile recreation, mailbox repair, or cache clear
- Verified mail flow is restored by sending a test message and observing client sync
- Documented the resolution for future tickets with the same symptom
Risks & Considerations
Warnings and gotchas
- Do not delete the OST file blindly. If the user has local-only items (rare, but possible with some add-ins or PST imports into cached folders), rebuilding the OST will lose them. Back up the OST before deletion.
- Removing and re-adding the Outlook profile is destructive to local signatures, rules stored client-side, and custom views. Export these first.
- Revoking sessions (if you suspect a stale token) will force re-authentication on all the user's devices, including mobile. Warn the user.
- Mailbox repair cmdlets (
New-MailboxRepairRequest) can take hours and generate mailbox I/O load. Run off-hours for large mailboxes. - Check tenant service health first. If Exchange Online is degraded, client-side fixes will not help and may confuse the user.
- Do not disable modern authentication or cached mode as a workaround without change approval — these are security and performance baselines.
Required Permissions
| Permission | Why It's Needed |
|---|---|
| Exchange Administrator or Global Reader | Required to inspect mailbox properties, quotas, and mobile device partnerships |
| User.Read.All (Graph) | Read user profile and account status via Microsoft Graph |
| AuditLog.Read.All (Graph) | Review sign-in logs to identify authentication or Conditional Access failures |
| Mailbox Search / Mailbox Import Export (optional) | Required only if running mailbox repair or content search during diagnosis |
| Helpdesk Administrator | Required to reset password or revoke sessions if a token issue is identified |
The fastest way to get this done — just ask Dex. Copy the prompt below and paste it into your Dex conversation.
For IT Admins
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For End Users
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