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 diagnose Office activation conflicts programs or websites to be sure Outlook is the only problem.

Method 4: Recreating the Outlook Profile

Another which is shown below, is a user opens a new message, or replies to one, and lets it sit for a while. When they go back to the message and press send they get an error that the message has been deleted. Ive repaired the OST and changed cache Fix Outlook Not Opening After Windows Update mode to only cache 1 year.

Check the connectivity to your email server

• We have resources to help you identify potential issues and fix them.
• There are various methods to resolve the Outlook sync issues, including manual and third-party solutions.
• Before you begin, make sure that your device has the latest operating system and app updates.
• Below are the steps I’ve taken to try to resolve this issue.
• Firewalls and antivirus software might prevent you from syncing your email and calendar events.
• If so, you'll see a message that says, "There is a problem with the server's security certificate. The security certificate is not from a trusted certifying authority."
• This solution will be beneficial for users who are facing Outlook syncing issues because of corrupted PST or OST files.

You'll also receive a mail (to the alternate email and to your admin email) from Microsoft resolve Outlook not sending emails fast 365 that indicates your tenant has encountered directory synchronization errors. For details see Identify directory synchronization errors in Microsoft 365. I still have it turned on on my laptop and it has not shown any issues yet.

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

If users are facing syncing issues in Outlook due to corrupt Outlook data files, they must choose a professional tool, like Shoviv OST to PST Converter Tool. This software allows users to process numerous OST files at once without any size limitations. Moreover, it maintains the folder hierarchy and metaproperties of the data when processing Outlook data files.