The libevolution package is missing a Recommends or Suggests on the
highlight package. When /usr/bin/highlight is present, selecting the
mail message view context menu "Format As" submenu items causes the
message to use language specific highlighting (Python/Java etc). It
appears the text-highlight module implements this feature and that
module is available in the libevolution package, so the libevolution
package should have some relationship with the highlight package.