The Microsoft KB article is a little vague about the cause of the problem. They are worth keeping in mind but not get concerned about. Happily, in most normal situations you’re unlikely to get near these limits. We’ve heard from some readers who have struck the 500 folder limit after importing a lot of data from other sources. There’s no warning and no data lost – just a slow Outlook. When you get near or past these values, Outlook and Exchange Server will slow down noticeably.
The performance limits apply when using Cached Mode with an Exchange Server account or connect to a Shared Mailbox. (Microsoft hasn’t updated the information to cover Outlook 2016 for Windows.) See Outlook extremes – how big and long can you go with Outlook? for more details. In theory, Outlook can handle almost any number of folders or items but in practice there are performance limits, especially if you’re using Exchange Server / Office 365 hosting.
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