I subscribed to Office 365 and I am unable to sign in to Lync for Mac 2011. I've tried the following and it still doesn't work:
* I uninstalled Lync and installed it again
* I updated to the latest version of Lync
* I verified that my username/password are correct
* I checked on manual configuration and typed "sipdir.online.lync.com:443" in external and external server name
* I checked that there is no firewall
* I tried to login first with an incorrect username and then with the correct one
* I deleted the keychain associated with Lync
* I deleted the files on "Microfot Lync Data"
Thanks in advance for your help!