Returned from vacation on 9/5 and restarted my desktop (running Windows10 with Norton Internet Security). Have the latest version of Thunderbird. Now I can no longer send email messages (can receive msgs) and get the following error message: An error occurred while sending mail. The mail server responded: From address not verified - see http://help.yahoo.com/l/us/yahoo/mail/original/manage/sendfrom-07.html. Please verify that your email address is correct in your account settings and try again. Also sometimes my draft messages are not saved and appear to have been sent even though I received the error message. I followed the suggested steps to diagnose the problem but this did not help I also contacted AT&T regarding this problem but they cannot help to correct.