The ping manager rightfully makes an exception (=return true despite error) if the pinged entity replies with cancel/service-unavailable. However quite a few implementations will return cancel/feature-not-implemented instead. (ejabberd is an example but there are also a few clients which do (Conversations). I think it would make sense for the Ping Manager to accept feature-not-implemented as well. At least I don't see a downside.
Issue #113 resolved