This article provides reasons why you could receive the delivery failure message containing the 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 messages to Yahoo! from an third-party service.

When you see this error message in your SMTP logs (where x.x.x.x is your IP address), it’s because of either of the following:

• We are seeing unusual traffic from your IP address.
• Email from your mail server is generating complaints from Yahoo! Mail users.

Note: This is typically a temporary situation and we encourage you to retry sending email to our servers approximately four hours after you encounter this error.

If you see 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 at this page.

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.