Is that Outlook reporting that error code? If so I think that's the "suspicious activity" code. Yahoo thinks you are sending spam. Do you have a legacy account like sbcglobal.net?
Don't keep retrying to send email. You have to wait a while before you can send again assuming it ever lets you send again.
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We're here to help! Its a message indicating a suspicious activity. To get started, we advise that you check your internet connection, clear all the caches and cookies, try to change your browser, filter your email recipients and then send email without attaching any files.