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There are a few problems that users may get while using Yahoo. One of such troubles is Yahoo SMTP 550 error. If you have received the 550 requests failed mailbox unavailable Yahoo error, use DNS blacklists to prevent spammers from accessing your Yahoo email account. Make sure your anti-spam settings are configured correctly. Furthermore, you should verify your device's time and date settings, correct them if they are incorrect, and check your device's SMTP settings. In addition, you should verify that your mail account does not contain a virus or other bug and that you have no objectionable content in your email account.

See also - Resolve Yahoo SMTP Error Codes, Yahoo Mail Delivery Error 554