9.2.2 Treatment for Important or Unread Messages

If you specify any of remove, trash, a folder name, or a standard function, messages with marks in wl-summary-expire-reserve-marks (which are called reserved messages thereafter) are retained.

Per default, this variable include the important, new, and unread marks, so that messages with these marks are not removed. Note that you cannot include the temporary mark (i.e. temporary marks are removed anyway), and be sure to process temporary marks before you invoke expiration.