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Outlook Mobile Gets a New Architecture

Outlook Mobile

Outlook Mobile’s Simplified Architecture

Last week, Microsoft announced that they have made big changes to simplify the architecture used to connect Outlook mobile clients to Exchange Online. This change was flagged at the BRK2177 session at the recent Ignite 2018 conference.

Going back in time, in late 2014, Microsoft bought Acompli. In early 2015, Microsoft launched a rebranded Acompli client as Outlook for iOS and Outlook for Android. The new clients solved a big problem in Microsoft’s mobile email connectivity strategy, which up to that time was busily going into the dead-end called OWA for Devices.

Acompli’s highlight feature was the Focused Inbox, which is now available in all varieties of Outlook. To make features like the Focused Inbox work, Acompli relied on its own API. Clients connected to Exchange with Active Sync and the mailbox data was transformed by services running on AWS before being consumed by the clients.

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From AWS to the Microsoft Cloud

In Fall 2016, Microsoft dumped the AWS infrastructure and moved Outlook processing to Azure. The clients persisted with their own API and the gap between the Acompli heritage and the Microsoft Cloud was bridged by a stateless protocol translator. In effect, a component that translated calls into a format that either side could understand.

Stateless No More

Now, Microsoft is replacing the stateless protocol translator and replacing it with some home-grown synchronization technology using HTTP connections. According to Microsoft, the synchronization technology “is a stable data sync protocol that has been used by the Windows 10 mail client for the past several years.”

Outlook Mobile Architecture
Figure 1: The old (top) and new (bottom) synchronization architecture for Outlook mobile (image credit:Microsoft)

Microsoft says that the simplification will reduce latency, which might help speed up delivery of email to devices. It also reduces the number of protocols in use within Office 365. It also allows Outlook Mobile to be supported in some of the Office 365 instances dedicated to high-security customers. Connections use modern authentication (OAuth) where clients get an access token by authenticating against a directory provider like Azure Active Directory and then use that token to retrieve data from Office 365. Among the advantages of modern authentication is that the user’s password is never sent to Office 365.

New Features Coming

Apart from simplifying the Outlook mobile architecture, the new platform is the foundation for mobile clients to deliver much-desired features like support for shared mailboxes and S/MIME. In passing, you can connect shared mailboxes to Outlook mobile today, but the method depends on IMAP4, and the rights management-based Encrypt feature allows organizations to move away from S/MIME to a more modern encryption scheme.

Rolling Out Soon

Microsoft says that users began using the new architecture in October 2018. Because and Exchange Online share the same infrastructure, a high degree of confidence exists that the roll-out will be smooth when it starts for commercial tenants in early December 2018.

Four years after the acquisition, there’s no doubt that Outlook mobile is a huge success for Microsoft. Before Acompli, Exchange users tended to use the native email clients on mobile devices, like the iOS mail app, with all the joy and heartbreak that involved.

Today, Outlook Mobile is the de facto standard for mobile email connectivity to Exchange Online and Exchange on-premises.  The functionality available to users in the Outlook mobile clients has grown over time. This change will see Outlook mobile progress even more, and that’s a good thing.

For more information, see the writeup about the architecture and FAQ posted by Microsoft online.


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Tony Redmond has written thousands of articles about Microsoft technology since 1996. He covers Office 365 and associated technologies for and is also the lead author for the Office 365 for IT Pros eBook, updated monthly to keep pace with change in the cloud.