A Smart Mailbox displays in one location messages that are stored in other mailboxes, based on criteria you define, so you don’t have to physically move messages between mailboxes.
When you change a message that you’re viewing in a Smart Mailbox, such as marking the message as read or unread, or moving or deleting the message, the change is reflected in the mailbox where the message is actually stored.
A Smart Mailbox automatically updates its list of messages to include new messages that match the criteria you defined.