MSN offlline messages failed

I don't use much msn so I don't know if it's normal for this protocol or not: when I want to send a message to someone who is not in my buddy list(and it seems that the person is offline) I get this message:

Message was not sent because the system is unavailable. This normally happens when the user is blocked or does not exist.

And I know for sure the person really exists and I am not blocked. Don't you know what to do about it?