The IDLE Command
The IDLE feature allows supported IMAP accounts to receive new messages automatically without having to be told to check for messages. This is why you might see messages appear in Postbox very soon after it reaches the server. However, some servers do not support IDLE since it keeps the connection open by periodically sending a message to tell the server that "I'm listening."
In Postbox 6 IDLE is always "on" and cannot be changed.
In Postbox 5 and below, IDLE can be disabled by navigating to:
Mac: Postbox > Preferences > Accounts > [your IMAP account]
Windows Tools > Options > Accounts > [your IMAP account]
Then click the Advanced button.
In the subsequent window, uncheck "Use IDLE command if the server supports it."