For those who want/have already set a restriction from your internal network to Microsoft cloud services based on IP addresses, you must be aware that IP addresses used by these services (Office 365 and Microsoft Azure) are changing on June 6th.
As this is always difficult to follow these changes – the recommendation if you want to set a restriction is to use DNS filtering instead of IP filtering, you can subscribe to the RSS feed published by Microsoft where they will announce before the update – hopefully this will be updated for future changes too: http://onlinehelp.microsoft.com/en-us/office365-enterprises/o365ip.rss.xml
Also as reminder you can find at http://technet.microsoft.com/library/hh373144.aspx all the information regarding URL’s and IP addresses used by Office 365 services (including Yammer)