Depending on whether you expect IM to work like physical mail or a phone call, this may be the expected behaviour.
It seems the majority here seem to be expecting the former, although I think of IM as more like the latter: if the recipient is not available, then the message is simply dropped, much like I can't call someone who is not answering the phone.
It seems the majority here seem to be expecting the former, although I think of IM as more like the latter: if the recipient is not available, then the message is simply dropped, much like I can't call someone who is not answering the phone.