Customize Login window for exchange server

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C#

ASP.NET

Exchange-Server

Hi,

I am using two mail server.
MS Exchange Server
Zimbra Mail Server

both have different user names.

My company needs to develop a webpage as login page.If user of Excahnge server login through this page then he is redirected to Excahnge mail server without asking credential again.
and

if he is login as Zimbra user with zimbra id then he is redirected to Zimbra mail without asking credential again.

Is it possible?

if yes then How?
Please help.