If you have access to shared mailboxes other than your own, you may find that Outlook starts to run slowly or the shared mailboxes don’t sync properly.
By default Outlook downloads data for all mailboxes you have access too, which is good for quick access to data, but can be problematic with shared mailboxes.
You can switch this option off for shared mailboxes to see if this performs better for your own use case – to do this:
Open Outlook
Click on File > Account Settings > Account Settings
Double click on your primary email account and then click on More Settings button
Go to the Advanced tab and untick the option for Download shared folders under “Cached Exchange Mode Settings”
