Unsubscribe addresses with characters like + in them need to be url-encoded if they are going to be used in a query string. Not sure what the ? to & replacement was doing (was it broken?), but it is possible I broke something by removing it.

trunk/squirrelmail/plugins/listcommands/functions.php

@@ -95,7 +95,7 @@ function plugin_listcommands_menu_do() {
             } else {
                 $url = "plugins/listcommands/mailout.php?action=$cmd&";
             }
-            $url .= 'send_to=' . str_replace('?','&', $act);
+            $url .= 'send_to=' . urlencode($act);
 
             $links[$cmd] = makeComposeLink($url, $fieldsdescr[$cmd]);