Cannot see presence status or send instant messages when OWA is integrated with Lync Server 2013 instant messaging

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Lync Server 2013 More...Less

Symptoms

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.

Cause

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.