[ticket/17299] Allow event to modify email core vars before sending a message

PHPBB3-17299

phpBB/includes/functions_messenger.php

@@ -582,6 +582,11 @@ class messenger
 		);
 		extract($phpbb_dispatcher->trigger_event('core.notification_message_email', compact($vars)));
 
+		$this->addresses = $addresses;
+		$this->subject = $subject;
+		$this->msg = $msg;
+		unset($addresses, $subject, $msg);
+
 		if ($break)
 		{
 			return true;