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How to deal with caching for shared mailboxes in Office 365


One of the most common things customers need to consider when migrating to Office 365 is the size of the user and shared mailboxes, which needs to be cached in an FSLogix Container to get the local native experience back to the user. In most cases, the old on-premises Exchange environment had no sizing restriction, which causes large mailboxes of sizes up to 20 GB or even more.

Caching the user mailbox is something that can be done very quickly with Outlook 2010, 2013 and 2016, but with shared mailboxes, there are some difficulties. Outlook 2013 and lower versions can either cache the entire shared mailbox or work only in online mode. Unlike user mailboxes, there are no retention configurations possible for shared mailboxes.

I’ll explain the most important things you need to be aware of when you are in some situations where you have to decide (proactively) how to take care of large mailboxes in your environment when using FSLogix Containers.

What are Shared Mailboxes?

Shared mailboxes in Exchange Online allow groups of your end users to manage a single mailbox for communicating with customers, partners, etc. In this article, I’ll show you how shared mailboxes work and how to manage them with Office 365 and the different versions of Outlook in your end-user environment, such as Citrix XenApp and XenDesktop, VMware Horizon, RDS and physical PCs as well.

Using Shared Mailboxes – Determine the problem

You probably recognize the menu in the image below as the place where you can activate Cached Exchange Mode for shared (mailboxes) and Public Folders. But, there isn’t a slider available over here, which makes it very hard to control the amount of storage that will allocate within the FSLogix Office 365 container on the file repository location. When you activate the option, the entire shared mailbox will be cached into the FSLogix container. So, for example, when the This email address is being protected from spambots. You need JavaScript enabled to view it. shared mailbox is 20 GB large, the OST file in the FSLogix container data allocation will be 20 GB as well!

Note: Mailboxes can be added through the Advanced - Microsoft Exchange menu as well as automatically through Exchange. When a user is assigned in the Mailbox Delegation - Full Access rights of the shared mailbox, the mailbox will be automatically assigned to Outlook. When there is an Active Directory group attached, you’ll need to enter in the mailbox manually for the user, just like the picture below. The simplest way is to set the user directly in the Full Access rights so Exchange will map the Shared Mailbox automatically to the user’s Outlook profile.


How to solve the cache problem for Shared Mailboxes?

Cache mode - user mailbox

Cache mode - shared mailbox

Outlook 2007

Yes, only for the entire mailbox

Only the complete shared mailbox

Outlook 2010

Yes, only for the entire mailbox

Only the complete shared mailbox

Outlook 2013

Yes, including slider with retention of 1, 3, 6, 12, 24 months and entire (all) mailbox.

Only the complete shared mailbox

Outlook 2016

Yes, including slider with retention of 3, 7 14 days and 1, 3, 6, 12, 24, 48, 36, 60 months and entire (all) mailbox.

Yes, cache mode aligns with slider setting of user mailbox. Note: Only available for Outlook 2016

There are three methods to work around this limitation of Outlook 2013 or older clients.

I’m going to explain all 3 of them over here. So, you can decide which one fits best for your organization.

  • Use Outlook 2016 in your environment
  • Make use of Online-Archiving in Office 365
  • Configure Shared Mailboxes as extra Outlook Profile

Decision diagram

 Use the following decision diagram to help choose which solution fits best for your organization.

 Image 2 decision diagram

How to use Outlook 2016 in your environment

The Outlook 2016 cache size slider also controls content from shared mailbox accounts. Users of Outlook 2013 (and older versions) will have to use other methods to control the impact that a shared mailbox account will have on their offline cache, such as using Online Archiving or separate email accounts in your Outlook profile.

There is, unfortunately, no graphical slider for shared mailboxes like there is for user mailboxes.

Please use the following Microsoft Support article as follow up if you have extra questions.

Image 3 Change Account

There will be 1 OST file created in the container for both the User and Shared Mailboxes!

How to use Online Archiving in Office 365

Exchange Online Archiving offers users advanced archiving capabilities with the archive mailbox feature. An archive mailbox is a specialized mailbox that appears alongside the users’ primary mailbox folders in Outlook or Outlook Web App. Users can access the archive in the same way that they access their primary mailboxes. Also, they can search both their archives and primary mailboxes.

Online Archiving is included in every Office 365 license and moves emails, calendar items, notes and tasks. It is one of the three approaches if you’re using Outlook 2013 or lower to deal with large shared mailboxes. It can be used for Shared and User Mailboxes as well.

For example, when you activate Online Archiving with a retention policy enabled for six months, all the items older than six months will move to the Online Archive. The mailbox size will decrease, and as a final result, the OST file will shrink as well.

Note: Online Archive mailboxes cannot be cached to the OST, so everything that you’ll move to the Archive works in Online Mode – and remember that you can use Online Archiving for User Mailboxes as well.

Note: Online Archiving is not the same as the AutoArchive option in Outlook, which migrates Outlook items directly into a .PST file. Learn more about Online-Archiving here and here.

Go to

Open the Admin console

Image 4 Admin Console

Click on Admin Centers -> Exchange

Image 5 Admin Centers 

Now we entered the Exchange Admin center.

Click on Compliance Management

Image 6 Compliance Management 

Click on retention tags

Choose for one of the 2 methods

Applied automatically to entire mailbox – means that all the Outlook folders will be part of the retention period. So all the custom folders, agenda items, notes, tasks all older than the time you specified will be copied to the Online Archive.

Applied automatically to a default folder – Only the folder you select is part of the retention period. This can be for example the inbox, junk email, calendar 

Image 7 Retention Tags

The differences between personal and default retention types are:

Personal – Retention policies that your user can activate within Outlook. The user is completely in charge of these policies.

Default – Outlook Items and mails older than the retention period will be transferred to the Online Archive. The process takes place on server level, so there’ll be no impact on the client side during this procedure. 

Add the retention tags that fit in your environment - and select the action Move to Archive

(I’ll use 6 months retention for the entire mailbox) 

Note: Move to Archive moves the item to the user's archive mailbox, if it exists. If a user doesn't have an archive mailbox, no action is taken. This action is available only for tags that are automatically applied to the entire mailbox (default) and tags applied by users to items or folders (personal tags).

 Image 8 New Tag Applied


Switch to the Retention Policies menu

Image 9 Retention Policies 

Now you find one Default MRM policy, which is applied to every user and shared mailbox in Office 365.

Note: You can use the default policy for your settings, but if you create multiple MRM policies with different retention settings. I would prefer to create a new one per department.

Click on the + button

 Image 10 button

Enter a random name for the retention policy, such as SharedMailboxesPolicy

Click on the + button

Image 11 new retention policy 

Select  the new created tag(s) and click on Add  followed by Ok

Image 13 select retention tags

Click on Save­­­

Image 12 new retention policy 2


The new retention policy is now ready to use. The last step is to assign the policy to the Shared Mailboxes.

 Image 14 Exchange Admin Center

Click on Recipients

Image 15 Exchange Admin Center 2

Click on shared

 Image 16 Exchange Admin Center 3

Open the settings of the shared mailbox 

Click on mailbox features and replace the Retention policy to the one we just created. Click on Save

Image 17 Edit Shared Mailbox

Now we need to perform the final step.

Enable the In-Place  Archive option of the Shared Mailbox.

Image 18 Exchange Admin Center 4

Click on Yes to start the archiving process

Image 19 Warning

As you can see, the status is now changed

Image 20 Exchange Admin Center 5

Click on View Details to see the status of the Online Archive

 Image 21 archive mailbox

You can now add multiple mailboxes to the same Retention Policy as in the previous steps.

The User’s Outlook will look like this after this change:

Image 22



How to configure Shared Mailboxes in an extra Outlook Profile

Note: When you use this approach, you’ll no longer need Full Access rights on the mailbox. You’ll need to enter in the account credentials of the Shared Mailbox. When you decide to do it, you’ll see the mailbox double in the mailbox list in Outlook as well.

This one is the easiest one, just add the shared mailboxes to the user’s primary profile of Outlook.

Note: Shared Mailboxes are free and don’t require an Office 365 license!

Click on File

Image 23 File

Open the Account Settings of the Outlook Profile

Image 24 Account Information

Click on New

Image 25 New 

Follow the steps of the  Auto Account Setup

Image 26 Auto Account Setup

Account is successfully added to the Outlook profile 

 Image 27 Add Account

Restart Outlook to activate the changes 

Re-enter the account credentials

Image 28 Windows Security 

Open the account settings again – you’ll now see 2 accounts active.

 Image 29 Account Settings

And more importantly – you can use the slider for the Shared Mailbox separately

Note: When you set the slider through Group Policy Management (GPOs) the retention will be taken over, so that works as well on this use case!

Image 30 Account Settings 2

There will be 2 separate OST files created in the container instead of one OST for both the User and Shared Mailboxes!