If you’re deploying Office 365 in your organization, you need to allow traffic from your users to Office 365. Indeed, all of your users will need Internet access to download their emails, chat on Lync, post on Yammer, and so on.
Microsoft gives us a list of domain names used by Office 365, and a list of IP ranges :
Note : IP range may change, Microsoft may acquire new public IP’s, and open new data-centers. It’s recommended to filter based on domain names.