If Outlook will not open, sync mail, or connect to the server, the issue is usually a profile problem, add-in conflict, network problem, or a damaged data file.
Outlook problems often come from a corrupted profile, a faulty add-in, an oversized mailbox, a damaged PST or OST file, or a temporary Microsoft service issue.
Launch Outlook without add-ins to see if one of them is causing the problem.
Turn off add-ins one by one if Outlook works in safe mode.
Make sure you have internet access and that Microsoft 365 or Outlook.com is not having an outage.
Rebuild the Outlook profile or repair the account settings if mail will not sync.
Run Quick Repair first, then Online Repair if the issue continues.
Clear old mail, large attachments, and cluttered folders so Outlook has less data to process.
If OST or PST data is damaged, create a new data file after backing up important mail.
If Outlook works on your phone but not on a computer, the problem is likely local to that Outlook install or profile.
Microsoft releases monthly patches that fix known bugs affecting Outlook's performance, sync reliability, and security. Enable automatic updates for both Microsoft 365 and Windows to ensure you always have the latest fixes. Many Outlook issues that seem like serious problems, such as search failures or sync errors, are resolved by installing a pending update that addresses a known server-side change or database corruption bug.
An oversized mailbox is one of the most common reasons Outlook slows down, freezes, or stops syncing. Archive old emails regularly and set up auto-archive rules in Outlook to move messages older than six months to a local PST file. Keep your primary mailbox under 10 GB if possible, and empty the Deleted Items and Junk folders weekly. A lean mailbox loads faster, syncs reliably, and is far less likely to trigger the connectivity or performance problems that frustrate daily email users.
Third-party add-ins for CRM, calendar integration, note-taking, and productivity tools are frequent causes of Outlook crashes and slow startup times. Open Outlook's add-in manager from File, Options, Add-Ins and disable any that you do not use daily. If you notice Outlook becoming sluggish after installing a new add-in, disable it immediately and test whether performance returns to normal. Less add-in load means less memory usage and fewer opportunities for a buggy extension to bring Outlook down.
Outlook may fail to open because of a corrupted profile, a bad add-in, or a damaged data file.
Outlook can stay disconnected because of a network issue, server outage, or account configuration problem.
Sync issues are often caused by a full mailbox, a damaged offline file, or account settings that need repair.
Use Microsoft Office repair tools on Windows, or re-add the account / rebuild the profile if needed on Mac.
Try the same service on a different device or network. If it works elsewhere, the issue is local to your device or network. If it fails everywhere, the service itself may be down - check Downdetector or the service's official status page to confirm.
Yes. A full restart (not just closing the app) clears stale connections, frees memory, and resolves the majority of intermittent glitches. Try this before deeper troubleshooting steps.
Open the App Store (iPhone) or Google Play Store (Android) and search for the app - if an Update button appears, tap it. On desktop apps, look for an About or Check for Updates option in the menu. Outdated apps often break after server-side updates.
If clearing the cache, updating, and restarting have all failed, reinstall the app as a last resort. A clean reinstall removes corrupted data, settings, and permissions that the standard fix steps cannot reach. Your account and saved data are stored in the cloud, so you won't lose anything by reinstalling.
A PST file is a personal data folder that stores your emails locally on your computer, while an OST file is an offline copy of your mailbox that syncs with the server. If Outlook cannot find or read the OST file, it may show disconnected or fail to open entirely. You can rebuild the OST file by closing Outlook, navigating to the file location, renaming it with a .old extension, and reopening Outlook so it creates a fresh sync copy. PST corruption usually causes missing emails or errors when searching. Use the built-in Inbox Repair Tool scanpst.exe to fix PST corruption without losing your data.
This looping password prompt is usually caused by saved credentials that have become stale or corrupted. Open Windows Credential Manager in the Control Panel, locate all entries under Windows Credentials that reference Outlook, Microsoft Office, or Microsoft 365, and remove them. Then restart Outlook and enter your password fresh. If the issue continues, verify that your account does not require app-specific passwords if you have two-factor authentication enabled, and check whether the Remember My Credentials option is checked in the sign-in prompt. Modern authentication issues can often be resolved by removing and re-adding the account in Outlook's account settings.