You get that message when you're trying to send an SMS text to a number who's not allowed to receive texts. Could be they only have a fixed number of texts per month allowed. Could be that number has had the SMS service blocked. Could be that number hasn't paid their bill. Could also mean that that number has had txts from your number in particular blocked.
I've even seen older phones have an issue where unless you specifically enabled the option to recycle old messages when your inbox is full, it would reject the txt until the user deleted msgs to free up space.
I don't mean to dig up an old topic, but I was googling this very event, came upon this forum, and decided to join seeing as I'm just recently a blackberry user (tour).
I've gotten about four of these messages in the last fifteen minutes, from a number I text frequently but have not texted in the last twenty four hours and have no messages sent to in my inbox. Any idea why?