Trying to share my experience in trouble shooting this problem. I notice that this problem occurs due to two reasons (i.e, if it was working earlier and for no ostensible reason starts giving this problem)
1. Yahoo Captcha code - Yahoo SMTP servers suspects your message to be spam and expects a code to be typed in. In this case you can log in to webmail account using browser and send the message. Captcha prompt will appear and you can input the code. Thereafter TB send will start working. 2. The message has non-text matter that TB yahoo webmail extension is unable to handle. You select Text only option from compose window (Options --> Format). Hope this helps. CRS
