Certificate issue on Outlook 2010

Hi,
 
We are experiencing issues with Outlook 2010. We receive a certificate error and Password prompts.
Certificate : “This CA Root certificate is not trusted. To enable trust, install this certificate in the Trusted Root Certification Authorities Stores”
 
Steps which has been tried to resolve this issue.
 
1.      Tried installing certificate to the local machine as well as on the user account selecting either options as Trusted certificate authority and automatically install certificate, it shows as the certificate has been imported but pops up with certificate error.
2.      Tried creating new profile though IMAP and it gives error as Incoming server error login failed error, but same settings were working before.
3.      Tried resetting user password in Exchange server but still prompts to enter password and doesn’t allow me to access Outlook emails.
4.      Tried configuring through exchange active sync but no luck.
5.      However, this works fine in Outlook 2016.
 
Please suggest what can be done to resolve the issue.
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