Consider the following scenario:
You deploy more than one front-end pool, such as pool A and pool B, in a Microsoft Lync Server 2013 environment.
You deploy Microsoft Exchange Server 2010.
You integrate Lync Server 2013 instant messaging with Exchange Server 2010 Outlook Web App (OWA).
You associate Exchange Server 2010 with pool A.
A user (user A) is homed in pool A, and another user (user B) is homed in pool B.
User A and user B sign in to OWA separately.
In this scenario, user A cannot see the presence status of user B. Additionally, user B cannot send instant messages to user A.
This issue occurs because Lync Server 2013 determines that pool A is not trusted, and that makes Lync Server 2013 refuse to route the ACK response message.
To resolve this issue, install the following cumulative update:
3001616 October 2014 Cumulative Update 5.0.8308.831 for Lync Server 2013 (Front End Server and Edge Server)