Defy xt not sending texts

I have a Motorola Defy XT on the old beta plan with unlimited talk, text & data. It always sent and received texts fine until a few months ago. Now it will only receive, not send, regardless of whether I am on WiFi or a cell tower. I can type them and tell it to send, and they appear to send, but the intended recipient never receives them. I am not finding anything in the settings that could be causing the problem.

Message an
Expert customer