After migrating to Outlook 2016 from 2010, I noticed this inconsistency.

If you use secondary mailboxes in Outlook, you’re probably going to want them in Online Mode rather than Cached Mode. With Cached Mode on, you’ll have an OST file created for each extra mailbox you add, and you’ll hit performance issues if you have over 500 folders over all mailboxes added to the account.

One of the ways to avoid these performance issues is turning off ‘Download shared folders’ in the mailbox settings:

‘Download shared folders’ disabled

This can be done manually, or company wide with the Group Policy setting “Disable shared mail folder caching” found in User Configuration / Administrative Templates / Microsoft Outlook 2016 / Outlook Options / Delegates. Enabling this will disable and grey out the option as per the screenshot above.

However, I was previously doing this through a registry setting ‘CacheOthersMail’ under HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\cached mode with the value set to 0. This worked on Outlook 2010 fine I believe, but in 2016 it did something slightly strange. Although clicking on a secondary mailbox’s folders showed they were in Online Mode with the status bar status of ‘Online’, the ‘Download shared folders’ tickbox was still enabled. I’ve confirmed this on both CTR and MSI versions of Office 2016.

At first I thought nothing of this, as it seemed to be working as intended. However, after a while I worked out that having it configured this way lead to performance issues, and people who had over 500 folders had cases where the inbox would stop updating. Changing the tickbox setting resolved the issue, despite the secondary mailboxes before and after this showing as ‘Online’. I didn’t dig into this any further so I can’t explain what was actually going on, but at a guess it was still doing some sort of sync or connection on each folder despite it being in Online Mode.

My advice is – make sure the ‘Download shared folders’ tickbox is off rather than just checking that the folders show as being ‘Online’. If you really need a secondary mailbox in cached mode but want to disable it by default, you could add it as a seperate mailbox account which will have it’s own cached mode settings.