Hi,
Are you using Outlook in the same internet environment? If not, please make sure the user have Outlook Anywhere enabled. See:
https://support.office.com/en-us/article/Use-Outlook-Anywhere-to-connect-to-your-Exchange-server-without-VPN-ae01c0d6-8587-4055-9bc9-bbd5ca15e817
Can the user log in to OWA successfully? What if we manually enter the server settings when configuring the Exchange account? Please contact the Exchange admin if you don't know the correct server settings.
If still no luck, please also try using the Remote Connectivity Analyzer to diagnose connectivity issues directly from the user's pc:
https://testconnectivity.microsoft.com/
Please let me know the result.
Regards,
Steve Fan
TechNet Community Support