The most common cause of this problem is a broken EAS sync partnership. Because the mail server is broken, troubleshooting the local Mail application will yield no results. Fortunately, you can manually reset the EAS sync partnership for an Outlook.com, Exchange, or Microsoft 365 Exchange Online mailbox using Outlook on the Web or Outlook Web Access if you have an Outlook.com, Exchange, or Microsoft 365 Exchange Online mailbox (OWA).
- In the Windows 10 Mail app, the EAS sync error 0x86000c09 indicates a sync partnership issue that you can fix yourself.
- Start Outlook.
- Click Accounts from the Tools menu. The Accounts window will appear on the screen.
- Click the minus sign (-) below the Exchange Account you want to delete in the Accounts window. A window titled 'Delete Warning' will appear.
- To conclude, click Delete in the Delete warning popup. Your Exchange account has been successfully terminated.
