However, this recommendation has changed as you also will see reflected on the TechNet page listing the required URLs or IP address ranges in order to access Office 365 and associated services. In the past, we used to split the documentation into datacenter sub-sections listing required URLs and IP address ranges for US, Europe, APAC etc. but these subsection are no longer to be found. Not the recommendation is to allow all URLs and IP addresses associated with any Microsoft datacenter hosting Office 365 and the associated services.
Why?
The answer is simple really. With wave 15, which is the latest Office 365 version, most of the URLs used to acces Office 365 now uses Geo-DNS, which means that if you have an organization with users travelling between regions, the user will use the closest servers available.
Let’s imagine you have an Office 365 tenant in the Europe region and one of the users travels to the APAC region. When that user connects to his mailbox, he will be serviced or more precisely be proxied through Client Access server (CAS servers) in the APAC region to his mailbox stored on a Mailbox server in Europe.
The above design change (proxy logic improvement) has been made in order to provide users with the optimal experience, which really is to provide a fast and responsive connection to his mailbox.
Until later,
Henrik Walther