The New Outlook Masters the Art of Offline Access

Microsoft added the initial support for offline working to the new Outlook for Windows in June 2024. Now they’ve announced (MC907098, 8 October 2024) that the client can start up when not connected to the internet. Truly this announcement deserves a roll of drums because it comes just 27 years or so after the Outlook classic client figured out how to start up without a network connection.

To be fair to the new Outlook, we live in an ultra-connected world and clients do expect to have connections to cloud services like Exchange Online. That wasn’t the case in 1997. The nature of early Wi-Fi networks and spotty dial-up phone connections often meant that Outlook 97 had no option but to work offline. Isn’t progress wonderful?

Microsoft 365 roadmap item 414516 refers to the feature as “offline app boot” and explains that offline access was previously only supported when Outlook was online and then went offline. My guess is that the Outlook developers hadn’t built the code to handle the detection of a lack of network and an elegant switchover to the local cache, which is what seems to have been added in the MC907098 update.

Details of what functionality is available when the new Outlook works offline are in this support article. General availability is expected in early October 2024 with full worldwide deployment complete by late November 2024. The update hasn’t reached me yet (version 1.2024.1002.100) as I still see the error in Figure 1 when I start the client without a network connection.

Control Over Offline Access

The ability for the new Outlook client to work offline is enabled by default. The Offline section of Outlook settings (Figure 2) allow users to decide if the client should be able to work offline.

Exchange administrators can block offline access by updating the OfflineEnabledWin parameter in the OWA mailbox policy assigned to user mailboxes. For example, this command blocks offline access with the new Outlook for every mailbox assigned the NoOfflineAccess OWA mailbox policy:

Set-OWAMailboxPolicy -Identity “NoOfflineAccess” -OfflineEnabledWin $false

To block offline access for everyone, run:

Get-OWAMailboxPolicy | Set-OWAMailboxPolicy -OfflineEnabledWin $false

Where does the New Outlook Cache Offline Data

Outlook classic spawned a cottage industry of ISVs specializing in creating products to do all sorts of things to the OST (offline storage) and PST (personal storage) files used by Outlook classic to enable that client to work offline. I receive frequent requests to endorse OST/PST products, none of which I accept.

The new Outlook doesn’t currently support PSTs. This feature is “coming.” The client doesn’t use OST files to create slave replicas of online mailbox folders. Microsoft hasn’t published any documentation to say where the local cache used by the new Outlook is, but some judicious poking around on a PC reveals that the likely storage location is in %LocalAppData%MicrosoftOlkEBWebViewDefaultCacheCache_data.

WebView is an important component for the new Outlook (and OWA), so the location makes sense. Monitoring what happens in the folder, we see (Figure 3):

A set of four “data” files (data_0, data_1, data_2, and data_3). I believe that these files form the offline cache read by the new Outlook.

A set of log files (like f_000094) that grow over time as the user works with Outlook. I believe that these files capture transactions such as new messages arriving, item deletions, and message sends. As part of client startup, a new log is created. When the client shuts down, any uncommitted transactions are written from the log into the data files.

An “index” file. This likely keeps track of the transactions captured in logs that have been committed to the data files.

There’s nothing particularly strange here. The same kind of arrangement is used by many database systems, including the Exchange JET Blue database engine.

If you delete all the files in the Cache_data folder and restart the client, Outlook creates a complete fresh set of files and populates them. You can’t delete the data or index files when Outlook is active because they are locked by the client. However, you can remove the log files and Outlook will continue to create new logs without interruption.

The above is my interpretation of what’s happening based on observation. I have no idea if it is 100% correct.

The Long March Continues

Anytime I write about the new Outlook, I have to remind myself that we’re in the middle of a long march to the point where the new Outlook can take over from Outlook classic. Microsoft has committed to supporting Outlook classic until at least 2029. At the current pace, they might need all that time.

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.