Are you access this through the Yahoo web interface, or via an application like Outlook or Windows Live Mail?

If you're doing it through the Yahoo web interface, you'll need to check your security settings there. Conversely, If you're doing it through an application like Outlook or Windows Live Mail, you'll need to check your security settings in that application. I could help with the latter two, but I've never used Yahoo mail or its web interface, so I can't help you with that.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]