I've logged in to Skype using my Live account and all my Live Messenger contacts were properly imported.

Some of them are Live Messenger contacts and some of them are Yahoo Messenger contacts and I can properly IM with them using Live Messenger or the Live Messenger web page.

However, I cannot send them instant messages using Skype. Any IM sent to them is not displayed to them and they always appear to me offline.

What can I do to be able to use Skype to send IM to my Live and Yahoo Messenger contacts?