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If you can sync your email when the firewall is turned off, you'll need to allow the Mail and Calendar apps through the firewall. Windows Defender Security Center is free security software that's included with Windows 10. If you haven't installed alternative security software, you should temporarily turn off Windows Defender Security Center. Test other clear Outlook credentials manager programs or websites to be sure Outlook is the only problem.
Method 4: Recreating the Outlook Profile
However once I joined this computer back to the domain and let it sit for a few days it was also working fine. Once the client started using Outlook again after some time has passed the issue had come back. I’ve tried to set cache mode from 1 year all the way down to a few days. If your password isn't the problem, you might have a certificate error. If so, you'll see a message that says, "There is a problem with the server's security certificate. The security certificate guide to Office 365 sign-in errors is not from a trusted certifying authority."
Check the connectivity to your email server
You'll also receive a mail (to the alternate email and to your admin email) from Microsoft 365 that indicates your tenant has encountered directory synchronization errors.It is possible for a corrupted Outlook profile to impede synchronization.If you're using an Outlook.com, Microsoft 365, or Exchange account, you won't have advanced mailbox settings unless the account was set up using advanced setup.Here is the strange part I tried to remove the computer from the domain and have Outlook run from a local account for a few days.
However I do not use Outlook on it as much as I do on my desktop. If you're using an Exchange account that requires advanced settings, you'll need to remove the account and add it again using advanced settings. To add the account using advanced settings, see Add an account using advanced setup. The Support and Recovery Assistant, a tool provided by Microsoft, can identify and resolve common Outlook problems. It is capable of automatically repair PST file without data loss identifying problems with account setups, sync, and other typical mistakes.
Methods to fix Microsoft Outlook syncing error
I ran the PSTscan tool to check for anomalies on the OST file (which it found and repaired but the issue still occurs). However, with one particular client no matter what changes are made these sync issue still occur. We're sorry you're having problems with the Mail and Calendar apps for Windows 10. To fix the issue, please try each of the following solutions in order.
Microsoft Outlook Sync issues
To make sure there are no issues, click Test Account Settings to test the connection. It is possible for a corrupted Outlook profile to impede synchronization. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. If the problems do not occur in safe mode, its most likely an add-in causing grief. I’ve been in contact with Microsoft they had me migrate his account\data to another server this did not fix it.