IMAP IDLE is an extension of the IMAP protocol to allow the client (Outlook, Apple Mail, Thunderbird etc.) to receive real-time notifications of mailbox changes without the need for the client to keep checking the server for new messages.
While this does provide real-time notifications for changes, it does have limitations. For example, you can only receive updates for the folder you currently have selected / viewing in your email client. IE if you have the “Inbox” folder selected and you receive an email in a folder called “Job Listings” the email client will not be updated until it does a standard refresh / check for new messages.
The reason for this is due to how IMAP IDLE works. When you select a folder, the client establishes a connection to the server’s IMAP service and states that it supports IDLE and the connection remains open and constant to the server. So imagine if you had 25 folders… That would be 25 connections to the server. That may not seem like much, but do that across thousands of IMAP email clients, some with 1 folder, others with 25, some with hundreds… You can quickly see this can become a connection nightmare for the server to handle.