Why cant i sent im text messages on aim 6.5?

When I go to send someone a text message on aim 6.5., it tells me the number (ex. +17749999999) is unable to recieve offline im's. In the settings, it's set to receive ims. I don't understand why it's giving me this error message. Sometimes it lets me send texts with no problems, and other times it gives me this error message and doesn't. All of my senders phones are on, so I don't know why it's doing this. Any thoughts?