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BT Yahoo! SMTP Security Upgrade - March 2008

As part of ongoing security enhancements to BT Yahoo! email, BT have upgraded their security to prevent emails from being sent from client applications (Microsoft Outlook, Thunderbird etc) when the "From" email address is not either a BT Yahoo! email address or a validated alternative email address.

If you see an error 553 message, you will need to validate the email address from which you wish to send. Step by step instructions can be found at BT Yahoo! Mail Help