Hello ,

I have this issue with user, where user connection to the Exchange server keeps dropping. the exact error log is: 

Connection to the Microsoft Exchange Server has been lost. Outlook will restore the connection when possible.

 When this happens I can ping the Exchange 2013 server sp1, it resolves correctly by IP and Hostname. Network is fine,  it's only connection to Exchange server.

Do you have a proxy server that intercepts HTTPS from the client?  Does the Exchange server have sufficient resources?

Hi,

For your question, I want to ensure all Outlook client will lose connection after this issue arise.

How about cache mode or online mode? Please run "Outlook.exe /rpcdiag" to check the connection for client with server.

This issue might be related to performance, please use Troubleshooting Microsoft Exchange Server Performance tool to check, for your reference: http://blogs.msdn.com/b/douggowans/archive/2007/01/02/a-very-quick-guide-to-monitoring-the-performance-of-your-exchange-server.aspx