OK, I finally got it working, 100%.
Only had problems when using Firefox browser. I switched to IE browser and have sent about 20 e-mails, so far, and they all went through. No more "failure notice" e-mails from "MAILER-DAEMON@yahoo.com".
When I was having problems, while using the Firefox browser, initially 100% of my e-mails were getting rejected. I was able to get some e-mails sent out by logging off the same e-mail account on my iPhone 4S. I was able to get about 50% of my e-mails sent out when I also logged out of Yahoo messenger. Go figure.
So, to repeat, no problems with error code 475 while using IE browser. And, while using IE, I have no problems being logged into the same Yahoo e-mail account on my iPhone 4S. I'm even logged on to Yahoo Messenger. Wow !!!
Hope that this helps,