I am sending an email from Microsoft Outlook 2010 (microsoft exchange) to a unix server. I am not receiving the email in server but I am getting the following error in outlook.
That error indicates that the domain in the email address -- y.com in your example -- is not resolvable by the exchange server. The problem is not with the unix server. The exchange server has to be able to "see" y.com.
If this is a test setup, add y.com to the exchange server's hosts file. Check the MS site for the location on your version of Windows, I'm not sure where it's located.
If a "live" situation, then y.com does not have the correct DNS setting to receive mail...no MX record in the zone - if that's the case, no one can send mail to it, not just exchange.