This article provides reasons why you could receive the delivery failure message containing the error "421 4.7.0 [TS02] Messages from x.x.x.x temporarily deferred - 4.16.56.1" when sending email messages to Yahoo! from a third-party service.

This error message indicates we are seeing unusual traffic from your IP address and/or that emails from your mail server are generating complaints from Yahoo! Mail users. Please note that this is a temporary error, and we encourage you to retry sending emails to our servers approximately 4 hours after you encounter this particular error.

If you are seeing this error consistently over a 48-hour period, and you are the administrator for the message content or mailing policies, we ask that you review outgoing messages for potentially objectionable content or practices.

Please know that Yahoo! is unable to disclose specific information regarding our filtering practices; however, we do publish recommended guidelines for your review.

If you are not the administrator of the mail server in question, please contact the administrator directly with the error message you are receiving.

We thank you for your understanding in this matter.