BT's full statement:

The email security enhancements we introduced earlier this month, although only affecting a minority of our customers, had a greater impact than we had expected. This is an important security enhancement needed to prevent spam and phishing abuse on the Internet however we rolled-back recent phases of this work as soon as we realised the impact it was having on customers and this will have cleared most of the recently-experienced 553 errors.

If you have use a different email address in the "From" field of your email client to that of your BT Yahoo! email address then it can still be used as long as the email address is not another BT or Yahoo! email address. This means that email addresses such as those of other email service providers (i.e. @tesco.net, @gmail.com, @tiscali.co.uk) or a personal domains can now be used in the "From" address field. This is a temporary state and we will have to deploy a fix to only allow this if these email addresses have been verified but we will only restart this change once we are satisfied the customer journey is as clear and simple as possible and we have communicated to our customers in advance.

Please note that users of Outlook 2003 SP2 or earlier (although now able to send emails) may still need to update their software to clear earlier errors:

Please click here if you're an Outlook 2003 user and still experiencing 553 errors: http://tinyurl.com/3dp5gy

If you're not an Outlook 2003 user, it is still possible that you are experiencing 553 errors, however you should be able to fix this easily by signing into your webmail and verifying your email addresses:

If you're not an Outlook 2003 user and you're still experiencing 553 errors, please visit http://www.btyahoo.com/verify

Please accept our apologies for the problems you experience and thank you to all users of these forums for your comments and suggestions. Despite the very understandable frustration many of you have expressed, we have also received some very useful ideas and feedback. The constructive remarks seen on the forum and newsgroups have been, and still are, useful to us to determine the best way forward, and serve to benefit others.