Office|Office 365

Outlook 2016 and its Many Connections to Office 365 Groups


Revealing Outlook’s Connections

Experienced Outlook users know that they can click on the Outlook icon in the system tray to reveal a menu of options. Instead of a simple click, using the CTRL+click combination forces Outlook to include a Connections option in the menu. In turn, this option exposes the network connections that Outlook has established to different resources, including the user’s mailbox, archive mailbox, public folders, and shared mailboxes. (and even site mailboxes, if you use them). This method works for on-premises and cloud servers and is a good way to find out to what resources Outlook is connected.



The Groups Connection

If you use Office 365 Groups, Outlook also lists the connections to your favorite groups and any other group to which Outlook has connected during the current session (Figure 1). My observation is that the number of connections can grow during a session to a point where a connection exists to every group to which you belong.

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Outlook Office 365 Groups
Figure 1: The many connections to Office 365 group mailboxes (image credit: Tony Redmond)

The question is what purpose do these connections serve? The answer lies in the nature of the way Office 365 Groups integrate with Outlook 2016.

Cached Mode Only

Outlook 2016 can only connect to Office 365 Groups when the client is configured in cached Exchange mode. In this mode, Outlook works with a cached local copy of the mailbox rather than depending on server connections. Times do exist when Outlook fetches information from the server rather than using cached data, but that’s usually only when the local copy is very out-of-date.

The local copy for a mailbox is held in an OST (Offline Storage Table) file. Cached local data for Office 365 Groups is stored in a separate file, the GST (Group Storage Table). The GST and the OST are found in the same folder (%HomePath%/AppData\Local\Microsoft\Outlook). Confusingly, the GST file has an .nst extension.

Outlook synchronizes the server copies of the inbox and calendar folders from group mailboxes to slave copies in the GST. The process is functionally similar to the “drizzle mode synchronization” used by every version of Outlook since Outlook 2003 to copy mailbox folders and their contents to the OST. Synchronization occurs via background threads that connect to the mailboxes and fetch new conversation and calendar items as they are posted to groups. It is these background threads that show up in Outlook’s connection status.

When an Outlook session is initialized, connections are set up for each group marked as a favorite. If you then click on the Groups section of Outlook’s resource list and expose the set of groups that you belong to, you’ll see that Outlook creates a new connection to the mailboxes for those groups to update their content.

Lots of nice Network Connections

The mechanism is quite logical when you think about it. Hopefully, a burgeoning mass of connections won’t cause any performance problems for users who belong to many groups. Microsoft hasn’t published any information on the topic, so feel free to do some experiments to discover where the outer limits lie. Maybe it is 100 groups or perhaps 200? Or even 500?

I can testify that I haven’t experienced any problems when Outlook creates connections for 50+ groups but I have not gone much past that point. My contacts in Microsoft say that they plan to update Outlook in the future to age out (or “decay”) connections when they are no longer needed. That sounds like the right thing to do. In the meantime, I wouldn’t worry too much about these connections – unless you know of a reason why!

Follow Tony on Twitter @12Knocksinna.

Want to know more about how to manage Office 365? Find what you need to know in “Office 365 for IT Pros”, the most comprehensive eBook covering all aspects of Office 365. Available in PDF and EPUB formats (suitable for iBooks) or for Amazon Kindle.

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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.
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