We are using Lync in a secondary college through the Education Department's Mail exchange. The PC's on the network all work seamlessly, but a Mac running OSX 10.7.4 will not connect, failing with the message "Sign in to Microsoft Lync failed because the service is not available or you may not be connected to the Internet". The Internet works on the machine and so does e-mail through the Exchange Server. The Lync server is set to automatic as are all the PC's. Any ideas?