Error "421 4.7.0 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.55.1" when sending email to Yahoo

Learn why this delivery failure error may occur (where x.x.x.x is your IP address) when you’re sending email messages to Yahoo from a non-Yahoo Mail address.


This error indicates Yahoo is seeing unusual traffic from your IP address and/or that emails from your mail server are generating complaints from Yahoo Mail users.

  • The error is temporary: we encourage you to retry sending emails 4 hours after encountering the error.
  • If you are not the administrator of the mail server in question: please contact the administrator directly to discuss the error.
  • If you are an administrator of message content and mailing policy: If you see the error consistently for 48 hours, review your outgoing messages for potentially objectionable content or practices.
    - See Yahoo’s recommended guidelines for bulk mail senders and postmasters: we do not disclose further information about our filtering practices.
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