As you now, Exchange 2013 does not use the CAS Array Name itself for connecting to Exchange, instead it uses the MailboxGUID (which is equal to the ExchangeGUID Attribute)
Connection Status of Outlook:
With the following command we can figure out to which server the Mailbox belongs to:
get-mailbox -Identity ex2013 | select servername, *ExchangeGuid*
ServerName : ex03
ExchangeGuid : 5fb1e055-c2b7-40c6-a953-be36da2b7863
Or just have a look to the “Proxy Server” in your Connection Status ;-) = right mouse + CTRL key on the Outlook Symbol in your taskbar.
have fun, Jimmy