This article describes how to configure integration between on-premises Microsoft Lync Server 2010 deployments and Exchange Online.
On-premises Lync Server 2010 supports interoperability with Exchange Online in the following ways:
- Calendar data and out-of-office message
- Instant Messaging (IM) and presence interoperability in Outlook Web App
- Voicemail interoperability
Outlook Web App can interoperate with on-premises Lync Server 2010 to provide users with instant messaging (IM) and presence within the Outlook Web App interface.
Read more at source: http://blogs.technet.com/b/nexthop/archive/2011/06/28/configuring-on-premises-lync-server-2010-integration-with-exchange-online.aspx