Any XMPP based server can communicate with MS Lync/Skype for Business Server IF the Lync/Skype deployment contains an Edge server. This is a server that resides usually in the DMZ and provides the federation to XMPP servers. The Lync/Skype Front End servers (located in the private network) actually have the XMPP gateway service. The XMPP is forwarded to the Front End servers from the Edge servers. This type of federation can use TLS, SASL, and Direct Server Callback depending on how you want to communicate.