Direct answer
Quick answer
If Outlook is not syncing, first confirm that Microsoft 365 is available, the device is online and the same mailbox works in Outlook on the web. Then check whether Outlook is offline, review recent password or multi-factor authentication changes and restart the application. Repeated or multi-user failures usually require an administrator to review service health, licences, sign-in logs and mailbox settings.
Key takeaways
Symptoms to look for
Look for slow mail sync, missing emails, or repeated password prompts.
Common causes
Mailbox settings, licensing, profile corruption, and authentication issues are common.
What fixes tend to work
Rebuild the profile, verify licensing, and confirm server connectivity.
Separate service, account and device faults
Before rebuilding anything, check whether the problem affects one message, one folder, one user, one device or the whole organisation. Confirm Microsoft service health where available, test Outlook on the web and compare another network or device. This narrows the fault without deleting a working profile or local data. Record exact error messages and times because vague descriptions such as “email is broken” make recurring faults much harder to diagnose.
Profiles, cached data and mailbox size
A damaged Outlook profile or local cache can cause slow searches, repeated prompts or missing synchronisation even while the mailbox works online. Confirm that server data is present before recreating a profile. Large shared mailboxes, excessive cached folders and add-ins can also affect performance. Avoid deleting data files until their purpose and synchronisation state are understood, especially where old Post Office Protocol files or local archives may contain the only copy.
When authentication keeps returning
Repeated sign-in prompts can be caused by expired sessions, incorrect saved credentials, device registration problems, licence changes or security policy. Check the account in a private browser window and review recent sign-in activity before reinstalling Office. If several users are affected at once, investigate tenant policy, service health and network controls. Escalate quickly when prompts follow a suspicious login or unexpected multi-factor authentication request.
FAQ
Why is Outlook not syncing?
It is often caused by account issues, cached data, or server-side mailbox problems.
When should I seek support?
If email stops syncing across devices or the issue keeps returning, support is recommended.
Official sources and further reading