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. This worked on Outlook 2010 fine I believe, but in 2016 it did something slightly strange. 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 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. One of the ways to avoid these performance issues is turning off ‘Download shared folders’ in the mailbox settings: 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. If you use secondary mailboxes in Outlook, you’re probably going to want them in Online Mode rather than Cached Mode. After migrating to Outlook 2016 from 2010, I noticed this inconsistency.
