"Outgoing Mail Server" refuses (again) to send email. Why?

Already, I have done the disclosures required to make my email "more secure" and now the refusal-to-deliver returns: ----
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The outgoing mail server “smtp.sbcglobal.yahoo.com:lawhawley@sbcglobal.net” failed to deliver this message. This message couldn’t be delivered because the SMTP host “smtp.sbcglobal.yahoo.com” did not recognized the [sentence actually end here]

If you would like to use a different delivery account for sending messages from this account for the time being, select it below and click “Try This Account.”
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What will correct this?