In Exchange 2013 you may of noticed that once you hit “check name” it underlines the users, however the server name is a displayed as numbers , then the @ sign then your domain name. Don’t worry, this is normal, Exchange 2013 now actually shows your GUID mailbox number instead of the server name.
Microsoft Outlook clients no longer connect to your servers fully qualified domain name like they used to. Outlook uses the Autodiscover service ( and the A record you create in DNS pointing to your CAS server ) to connect to your Exchange 2013 server. this is displayed as the mailbox GUID of the user, the @ symbol, and your domain name of the user’s primary SMTP address. So do not think that this is a problem, it is in fact by design.http://technet.microsoft.com/library/jj150540%28EXCHG.150%29