UPDATE – Outlook 2016 does respect the slider for Shared Mailboxes. You’ll see this referenced in the KB article linked below

By default, Outlook 2010 and 2013 cache the mail items in Shared Mailboxes. I’ve recently had a question from a colleague about solutions for using cached mode with very large (~10GB) shared mailboxes used by large numbers of users.

The problem the organisation is facing came to light after re-installing a number of machines as part of an upgrade to Outlook 2013, after a successful Exchange Online migration. Although the OST file was kept intact after the migration, further changes to a number of clients meant a full OST download ended up being necessary.

Whilst OST repopulation is a pain that many companies have to go through, the Slider feature in Outlook 2013 mitigates against the time taken to populate the OST by allowing the user (or admin, via GPO) to only download the latest month, 3 months, 6 months, 12 months or 24 months of mail – rather than downloading the lot.

However as explained in MS KB article KB2733062, this does not apply to shared mailboxes – which begin a full download on first use:

For organisations who rely heavily on very large shared mailboxes, switching off caching of shared mailboxes would be a typical option for an on-premises Exchange installation; however latency over the Internet to Office 365 mean that although valid as a temporary workaround it is not a good solution.

Possible workarounds

An ideal solution from Microsoft would be for Outlook’s Cached Mode slider to display the same behaviour for all mailboxes – but in the meantime, here’s a few suggestions that may alleviate these issues for end-users:

1) Add the Shared Mailbox as a separate account to the same Outlook profile, allowing you to configure the mail that is downloaded.

This makes use of the ability for Outlook 2010+ to have multiple Exchange accounts configured within a single profile, and takes advantage of Exchange’s ability for you to logon to a Shared Mailbox using the account that has been granted permission.

First things first – If the mailbox is auto mapped i.e. doesn’t show in this list:

 

 

 

 

 

 

You may need to remove and re-add the permissions using Exchange Online Powershell, specifying AutoMapping to be False:

Then after removing the Shared Mailbox from Outlook, add a new account to the single profile. Enter the Email Address for the Shared Mailbox, but NO password:

 

 

 

 

 

 

 

 

 

 

When prompted for credentials, ask the user to enter the same credentials they use for heir primary Office 365 mailbox:

 

 

 

 

 

 

After setup, choose “Change Account Settings” and alter the slider to an appropriate value for the Shared Mailbox. If you are using Group Policy to set the time period mail is cached for then your shared mailbox will now follow the same policy.

Of course this approach isn’t without downsides. Sent mail for the Shared Mailbox will be stored in its respective mailbox, and the user will also see notifications (particularly annoying in Windows 8/8.1) for the Shared Mailbox as well as their own. As workarounds go, this isn’t ideal – but it might help.

2) Configure Outlook to Download Headers Only, or Download Headers then Full Items

This option could allow users to download just the headers and reduce the OST download. This affects all accounts so has the same effect on their primary mailbox:

 

Although extreme, setting this option temporarily could get you out of a bind and is easy enough to guide users though over the phone.

3) Create a Retention Tag and Retention Policy for archiving. Apply the policy to the shared mailbox, then enable the shared mailbox to have an in-place archive.

This more extreme solution requires a light-touch on the clients, but is difficult to reverse once in place. We can use the same technique that we’d use in Outlook 2010 to limit how much data is cached locally by enabling the in-place archive (personal archive, online archive) and setting an appropriate retention policy. This policy will limit the amount of messages stored in the main shared mailbox, with older messages moved to an in-place archive mailbox.

First, go into the Exchange Admin Center and navigate to Compliance Management>Retention Tags, then create a new tag that is applied to the whole mailbox:

 

 

Select options to specify a period before it is moved to the Archive, e.g. 31 days:

 

 

 

 

 

 

 

 

After saving the new tag, we will then navigate to Compliance Management> Retention Policies and create a new policy for Shared Mailboxes, and then add the new tag we’ve just created:

Once this is created navigate to Recipients>Shared and apply the policy to a Shared Mailbox the new policy should be used against:

 

Finally, enable the  In-Place Archive for the Shared Mailbox:

The In-Place Archive should show in Outlook 2013 within an hour, or sooner if you close and re-launch Outlook. However, messages may not begin to move to the archive straight away. Expect messages to begin to move after a few hours.

Summary

Outlook 2013’s Offline Cache Slider is a great feature for limiting how much data needs to be downloaded. However for organisations using to Office 365 who are heavy Shared Mailbox users who perform PC upgrades or replacements this may cause issues as Shared Mailboxes may always be downloaded in full.

One would hope a future update to Outlook 2013 will address this issue, but in the meantime these suggestions may alleviate this behaviour.