Error: "421 4.7.0 [XXX] Messages from x.x.x.x temporarily deferred - 4.16.56.1"

If you're receiving any of the error messages listed below when sending email to Yahoo from a non-Yahoo Mail address it means Yahoo is seeing unusual traffic from your IP address. 

Temporarily deferred SMTP errors

  • 421 4.7.0 [TS02] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.56.1
  • 421 4.7.0 [TSS05] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.56.1
  • 421 4.7.0 [IPTS05] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.56.1

Reasons you will get this error message

Note: This traffic can result from things outside of the sender's control.

  • Using a shared IP address - Mail traffic from other domains could be negatively affecting your IP sending reputation. Contact your host provider to request using dedicated IP addresses to send your mail to resolve this problem.
  • Monitor your sender reputation - Even if you have a good reputation, users can vote your email as spam and affect your overall reputation. We review IPs submitted for delivery issues and, based on our review, we may modify your reputation in our systems if needed.

Fix the problem sending email to Yahoo Accounts

Review our guide to industry standards and best practices and consider the following when you're sending email to Yahoo Mail addresses:

  • URL reputation
  • Sender reputation
  • Domain reputation
  • IP address reputation
  • (DMARC) authentication
  • DomainKeys Identified Mail (DKIM) signatures