Answer
Check the plugin class referenced in the report layout file first — there are two similarly-named Excel plugin classes, one under a 'reportingservices' path (used for a different, simple variant) and one under a 'printagentservice' path, and for the classic Excel-plugin behavior on Apps8-era layouts you need the 'printagentservice'-path class specifically, not the 'reportingservices'-path one, or the plugin will silently do nothing. Also watch for an accidental leading space in that class-name string within the layout file, which likewise breaks it silently. Make sure a sender is also specified in the layout file, and when actually printing, don't tick the Email checkbox in the Print Dialog itself — instead print directly to a logical printer configured as NO_PRINTOUT, which is what triggers the Excel-plugin-based email path correctly.