This can be handy for those of you who are Federated with Microsoft :
” On June 8th, 2013, for Office Communications Server and Lync federation with Microsoft.com, Microsoft will replace the root certificate on the federation edge server from a GTE CyberTrust root to a Baltimore root.
By that date, any SIP domain federated with Microsoft must have the respective edge server’s trusted root certificate store updated with the Baltimore root certificate. Any edge server which does not trust the new Microsoft.com certificate by June 8, 2013 will be unable to connect to Sipfed.microsoft.com, thereby impacting federation with the Microsoft.com SIP domain. IM and presence will fail, as will A/V for any user within the federated SIP domain when attempting to reach a user in the Microsoft.com SIP domain. “
You can read the full details at :
Lync Server Federation with Microsoft.com: Root Certificate Change – NextHop – Site Home – TechNet Blogs