You may need to verify the account. This was the case for me. I had to sign
into web-based yahoo, go to mail setting and add an account
Hope it works for you
Jesper
This was message I got............
You've received an Error 553 message because we've upgraded your BT Yahoo!
Mail security to help prevent identity fraud and spam.
These enhancements will help protect you from 'spoofing' – when people use
alternate addresses to disguise an email's real sender, possibly to commit
fraud.
Making the improvements work
For the security improvements to work, we need you to take a few minutes now
to verify each of the alternate email addresses you use in BT Yahoo! Mail.
This will confirm that these email addresses are genuine.
Please verify addresses now to avoid any further blocking of your outgoing
emails.
Simply follow the step-by-step instructions at
www.btyahoo.com/verify.
You won't be asked for any personal information, but you'll need to log in
to your account to make the changes.