"Action required..." message to send emails

newb

When I try sending emails in yahoo on my iPhone I have recently started to get the following message:

"Action required. We need to check you're a real person. Please go to our Outbox and view your pending messages to comple the sending process."

I view the message, enter the code, hit send and am then given the "You're message was not sent" error. Returning to the Outbox then gives me the original "Action required..." message.

Any ideas why this is occuring? Any suggestions on how to keep it from continuing?

Thanks.

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