I can't send emails.?

I have been unable to send emails, but I can receive them fine. When I try to send a message I receive the following error message: "To prevent abuse, your message cannot be sent now. If the message contains only a link, please add some text to it. This helps us fight spam. If it still cannot be sent, please modify the message or try again later. Sorry for the inconvenience." I have heard of others having this same problem. Does anyone know what the issue is, or what I can do to fix it? Thank you!!