Exchange 2013 and later have changed the connection model such that connections to mailboxes are not required to be server-specific as was the case with previous versions. Exchange now allows any CAS role server to accept the connection from a client, and will redirect the connection to the server with the active database copy, which may not be the CAS to which the client connected. However, in order for a CAS to understand to which server to send the connection request, a hint was necessary in the servername.  This hint is seen as the GUID value on the left hand side of the @ sign.  This guid is actually the mailbox guid of the user’s mailbox (as seen in the LDAP attribute msExchMailboxGuid). When a CAS sees a server name in this format, it can quickly query the Active Manager to understand which mailbox role server is holding the active copy of that user’s mailbox database, and subsequently routes the connection to that server. So, this new format is to be expected for any connections to Exchange 2013, 2016 (or later versions), as well as Office365.