Federation trust - Both the on-premises and Office 365 service organizations need to have a federation trust established with the Azure AD authentication service.

So, to avoid confusion, we shall refer to both as the Azure Active Directory (AD) authentication Service, or Azure Auth Service for short. You may have heard of this referred to previously as the Microsoft Federation Gateway, and while speaking purely technically the two are quite different, they are different implementations of essentially what is the same thing. On-premises organizations configuring a hybrid deployment must have a federation trust with the Azure AD authentication service.

Azure Authentication Service - The Azure Active Directory (AD) authentication Service is a free cloud-based service that acts as the trust broker between your on-premises Exchange organization and the Exchange Online organization.

With the help of TechNet and other resources, here are some basic definitions

The first thing to do is to get some basic terminology clear. The aim of this post is to explain in more detail how this server to server communication works, and to help the reader understand what risks this poses, how these connections are secured and authenticated, and what network controls can be used to restrict or monitor this traffic.

This communication takes place over the Internet and so this traffic must pass through the on-premises company firewall to reach Exchange on-premises. In addition, a hybrid deployment can serve as an intermediate step to moving completely to an Exchange Online organization.īut one of the challenges some customers are concerned about is that this type of deployment requires that some communication take place between Exchange Online and Exchange on-premises. A hybrid deployment provides the seamless look and feel of a single Exchange organization between an on-premises Exchange organization and Exchange Online in Microsoft Office 365.

A hybrid deployment offers organizations the ability to extend the feature-rich experience and administrative control they have with their existing on-premises Microsoft Exchange organization to the cloud.

Exchange hybrid modern authentication