Since the exchange server was upgraded from Exchange 2016 to Exchange server 2016 CU8, users are facing 2 issues as below.
1. Existing users are unable to sync e-mail.
2. New users are unable to logon. 

 

In the logs we  see 
"2018-02-28T10:14:00.812+0100","a","INFO ( 4)","Got exception while trying to use nwstack: java.net.SocketTimeoutException: response timed out",6515,6763,Secure Mail, , ,0 
"2018-02-28T10:14:00.815+0100","MDX-SecureBrowse","CRITICAL ( 1)","Not hooking URL.openConnection(proxy)",6515,6763,Secure Mail, , ,0 
"2018-02-28T10:14:00.872+0100","a","ERROR ( 2)","Authentication failed",6515,6763,Secure Mail, , ,0 
"2018-02-28T10:14:00.872+0100","a","ERROR ( 2)","Authentication failed",6515,6763,Secure Mail, , ,0 
"2018-02-28T10:14:00.912+0100","UserAlert","WARNING ( 3)","Incorrect user name or password.",6515,6515,Secure Mail, , ,0 
"2018-02-28T10:13:17.790+0100","a","INFO ( 4)","Account SSO check: Unauthorised user, http response code : 401",6515,6631,Secure Mail, , ,0 
"2018-02-28T10:13:17.791+0100","a","ERROR ( 2)","Account SSO check: authentication method not supported by client, auth headers : ",6515,6631,Secure Mail, , ,0 
"2018-02-28T10:13:17.791+0100","a","INFO ( 4)","SSO Result : 3",6515,6631,Secure Mail, , ,0 

On exchange Server, Enable client certificate mapping for the Outlook on the Web and ActiveSync virtual directories.

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Problem Cause

The upgrade disabled the client certificate mapping setting on Exchange . It needs to be enabled if cert based authentication is being used.

This article summarizes the steps to follow if the scuremail is giving incorrect username and password for cert based auth enabled configuration