When migrating an organization from Exchange on-premise to Exchange Online, one network-integrated system that is commonly overlooked is web proxy filtering. If your organization has a web proxy in place, it is imperative that you review Microsoft’s URL and IP address ranges to ensure you have exceptions in place.
Without these exceptions in place, users in your environment will almost certainly run into access issues, including the inability to connect Office applications to Office 365. In many cases, your users will not be able to authenticate to Exchange Online from Outlook (using Exchange Anywhere). In some cases, users in your environment may experience significant bandwidth allocation issues, resulting in poor Outlook performance after go-live.
Microsoft provides an up-to-date list of all Office 365 URLs and IP address ranges, which you can access here: https://support.office.com/en-us/article/Office-365-URLs-and-IP-address-ranges-8548a211-3fe7-47cb-abb1-355ea5aa88a2 (If you are using Office 365 operated by 21Vianet in China, use the following URL: https://support.office.com/article/URLs-and-IP-address-ranges-for-Office-365-operated-by-21Vianet-5c47c07d-f9b6-4b78-a329-bfdc1b6da7a0)
The link above provides all URL and IP exceptions that will need to be created in your organization’s web proxy system to support Office 365, including the following components:
Office 365 Portal and Identity
Skype for Business Online
Exchange Online Protection (EOP)
Office 365 ProPlus, Office Online
Office Mobile / Mobility Products
With the correct exceptions in place, your users will benefit from a smooth, efficient Office 365 experience in your network.