With Outlook 2013 we saw the introduction of the Sync Slider, which helps limit the size of OST’s by controlling how much recent mail was synchronized locally on the device. For Exchange accounts, this was 12 months by default, with the option to go down to 1 month or up to all mail.
In Outlook 2016, Microsoft added more sync options that allow for better storage management by only retaining this much mail on the device:
• 3 days;
• 1 or 2 weeks;
• 1, 3, 6, 12 or 24 months;
As before, this setting can also be adjusted using Group Policy.