Sometimes Called Exchange Fast Access
In September 2012, I attended the Microsoft Exchange Conference (MEC) in Orlando. Later, I wrote an article about Outlook’s hybrid mode, which Microsoft shipped in Outlook 2013. The idea was to improve performance for Outlook when the client works in cached mode by allowing Outlook the option to make network requests to fetch data instead of depending on data synchronized to the OST. The feature works well on fast networks because Outlook can connect and display data like new messages waiting in the Inbox faster than if the client had to wait for background synchronization to finish.
All subsequent versions of Outlook (desktop) support and use hybrid connections when the client is configured in cached mode. The feature was called “Exchange Fast Access,” but according to Microsoft documentation, it was deprecated in Outlook 2016. Perhaps it was only the name that was deprecated. Exchange Fast Access doesn’t really tell you what the feature does. In fact, it’s all about forcing Outlook to depend on the cached data in the OST., which is how Outlook worked when cached mode appeared in Outlook 2003.
Why Disable Outlook Hybrid Mode
I was reminded about my 2012 article during a discussion in the Microsoft Technical Community where a contributor named Bill Rupp said that his organization disables hybrid mode to force Outlook to use the OST when connected over high latency networks. They also disable hybrid mode for clients used by frequent travelers, who often connect using flaky Wi-Fi networks. The reason? Well, they feel that Outlook hangs too often when the client runs in hybrid mode and is allowed to switch between network and local data. I can see why this might be the case as an attempt to fetch network data across an unreliable link is always prone to cause problems.
Controlling Local Caching
Microsoft introduced the LocalCaching registry setting in Outlook 2013, saying: “Enable setting to turn off Exchange Fast Access. This forces user accounts to access data from the local cache.” In other words, if you set the LocalCaching to 1, you disable the (deprecated) Exchange Fast Access feature and force Outlook to use data cached in the OST. By default, this value probably doesn’t exist on your PC, so you must create it before setting the value to 1. Here’s what I configured for Outlook version 1907 (click to run build 11901.20176). Usually, the best idea is to run the RegEdit utility to make registry changes, which is what I do.
HKCU\software\policies\Microsoft\Office\16.0\Outlook\Hybrid Value: LocalCaching DWORD: 1 (disable) 0 (enable)
After making the change and restarting Outlook, I noticed that Outlook’s startup behavior is slightly different even on fast networks where a distinct pause happens before new messages appear in the Inbox. I also noticed that Outlook reports some issues with search which means that search results are not as reliable as when hybrid mode is used. Apart from that, everything works as normal. Bill Rupp reports that the setting works for Outlook 2016 and 2019 (perpetual versions).
It might be that forcing Outlook to use locally cached data could be a solution for some of your users who operate in poor network conditions. Old solutions sometimes work well in modern conditions… even when Wi-Fi networks available now are so much better than they were in the past.
Insight like this doesn’t come easily. You’ve got to know the technology and understand how to look behind the scenes. Benefit from the knowledge and experience of the Office 365 for IT Pros team by subscribing to the best eBook covering Office 365 and the wider Microsoft 365 ecosystem.
Does this work in the Outlook 365 desktop application as well, using on prem Exchange? I have frequent flakey in-flight connections and this seemed like a possible solution to non-responsive Outlook, but I can’t tell if the setting is actually working for me.
It’s been four years since I looked into this topic. What I would do is test the setting while on a flight and see if it makes any difference.
I’m trying it now and experiencing intermittent UI lock-ups when network is flaky. I can’t tell if the registry setting has any effect or not — or perhaps it moved in O365, or was completely eliminated as it was deprecated in O2016. Wondering if there is anywhere I can look to see if it actually has an effect.
Short of doing a network trace to see what’s going on the wire at different times during a flight, I can’t think of how you’d test this.