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.
See Outlook extremes – how big and long can you go with Outlook? for more details.
The limits are:
Outlook 2010 and Outlook 2013:
- 100,000 items per folder
- 500 folders in the mailbox.
(Microsoft hasn’t updated the information to cover Outlook 2016 for Windows.)
Outlook 2007:
- 50,000 items per folder
- 500 folders in the mailbox
The performance limits apply when using Cached Mode with an Exchange Server account or connect to a Shared Mailbox.
When you get near or past these values, Outlook and Exchange Server will slow down noticeably. There’s no warning and no data lost – just a slow Outlook.
We’ve heard from some Office-Watch.com readers who have struck the 500 folder limit after importing a lot of data from other sources.
Happily, in most normal situations you’re unlikely to get near these limits. They are worth keeping in mind but not get concerned about.
The Microsoft KB article is a little vague about the cause of the problem. It seems that it’s an Exchange Server issue more than Outlook itself. However the references to .PST files (which are used for non-Exchange Server accounts) suggest otherwise.
Whatever the root cause, just avoid those very high limits and you’ll be fine.
