If the user has an Exchange Server 2003 mailbox, the Lync client will not be able to use EWS to interact with Microsoft Exchange.Rather, MAPI will be used in a fallback capacity to access the Microsoft Exchange mailbox.For example, the conversation environment feature leverages both Exchange Web Services (EWS) and MAPI to manage Conversation History items.Unlike previous versions of Lync, EWS is now the primary method used to provide Microsoft Exchange integration features for the Lync client.
In with that is a request for a list of folders in the exchange store.
While the Lync client will be able to write new archived Conversation History items into the user’s mailbox, it will fail to retrieve any existing Conversation History items from the user’s mailbox.
However, for a user whose mailbox resides on a server running Exchange Server 2007 SP1 or Exchange Server 2010, the Lync client will leverage EWS to provide the user with a complete experience for the conversation environment.
MAPI will be used if EWS is unavailable, but only in a limited capacity.
The impact of this design can be significant for a given user, depending on the location of their mailbox.
The first port of call for this kind of issue is the ETL log for communicator.