Here’s a pretty important message from your friendly neighbourhood Office 365 Message Center which could require some IT Pro action in your organization/Clients tenancies:

On May 16, you may encounter issues if you haven’t updated IP address list filtering for Yammer.

If your organization restricts computers on your network from connecting to the Internet, we have a list of the URLs you should include in your outbound allow list to ensure your computers can successfully use Office 365. URL allow lists are the recommended approach due to the modern web architecture of Office 365. For guidance on how to best implement URL or IP allow lists, please work with the manufacturer of the hardware or software you use to protect your internet connection.

The full updated URLs and IP ranges are detailed here:
https://support.office.com/en-ca/article/Office-365-URLs-and-IP-address-ranges-8548a211-3fe7-47cb-abb1-355ea5aa88a2?ui=en-US&rs=en-CA&ad=CA