mbox-hook [!]pattern mailbox
This command is used to move read messages from a specified mailbox to a different mailbox automatically when you quit or change folders. pattern is a regular expression specifying the mailbox to treat as a ``spool'' mailbox and mailbox specifies where mail should be saved when read.
Unlike some of the other hook commands, only the first matching pattern is used (it is not possible to save read mail in more than a single mailbox).