If you have deployed Exchange 2013 and Exchange 2016 then you will have users connecting to it one of two ways, either Outlook Web Access or an Email Client. If you use Microsoft Outlook you will be away of the section under Mail where you enter the saver name and the users account and click “check name”, the client goes off and checks that the user account is valid and connects Outlook to the mailbox. 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 and Exchange 2016 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