It sounds like you have already done a few of the steps, but I would still recommend reviewing the troubleshooting process found in the article below.
Thank you Griff. I have tried all of the troubleshooting ideas without success. Oddly, my problems started after I returned from a two week vacation, in which I took my computer along. Although I was able to restore the incoming mail, the outgoing just has never returned. Somehow, I believe this has something to do with my Yahoo account and their recent changes. I suppose I will wake up one day and the problem will have spontaneously resolved. I can only hope.