The Mail_IMAP::$fields property determines which fields appear in the 'in' and 'at' arrays of the Mail_IMAP::$msg property. The default fields are: 'fname', 'pid', 'ftype', 'fsize', 'has_at', 'charset' and 'cid'. Extra fields can be specified to make extra parameters available in the 'in' and 'at' arrays.