Having a large number of items in your Outlook folders causes notable performance issues. Learn about best practices for keeping your mailbox in control. DetailsBest practices to keep your mailbox lean:Organize your mail in folders and sub-folders - For optimal performance, Microsoft recommends that you do not exceed: 20,000 items for your primary defaults folders (See area 1 in the screenshot below), such as Inbox, Deleted Items, or Sent folders for example .100,000 items for your sub-folders (See area 2)5,000 items for your Calendar, Contacts, and Tasks (See area 3).500 folders in total (See area 4).