Recently, Free/Busy wasn’t working for onpremises users to Exchange Online users. Everything appeared correct and there were no glaring 4001 errors for Availability Service etc. Countless hours reviewing network settings, organizational relationship settings, exchange applications logs and everybody’s favorite outlook logging yielded not much. The proxy server for every CAS server was set through the operating system when the servers were built. I discounted this being the issue as the proxy was set through the operating system and that’s all I needed. After further troubleshooting and aggravation yielded nothing I decided to set the proxy through the Exchange Management Shell ‘Set-ExchangeServer servername -InternetWebProxy proxy server address:port’ on every CAS server not just the hybrid servers. Free/Busy instantly started working for onpremises users trying to view Exchange Online users Free/Busy.