If you've seen the above error when using your BT Yahoo Internet access you've probably been tearing out your hair trying to figure out why you can't send email using your your own domain name instead of the @btinternet.com or @btopenworld.com account. There are many people who have their own domain names which they use for sending email and BT in their infinite wisdom have decided that every single one of them will have to authenticate their email addresses. BT have not introduced this as a measure to reduce spam, but rather part of an "ongoing programme of security improvements" according to an article in The Register, BT customers are fuming. 

Solving the issue
Anyway if you received the error message you probably would have been taken to the following link which shows you how to verify your email address. But if you like me still got the error message, you've probably missed one important thing the article doesn't tell you. The email verification is case sensitive! That's right the email address has to appear exactly the same in verification as it does in your email program otherwise you will get the above error again.