Error: "421 4.7.0 [TS02] Messages from x.x.x.x temporarily deferred - 4.16.56.1"

Here are some reasons why you would get a "421 4.7.0 [TS02] Messages from x.x.x.x temporarily deferred - 4.16.56.1" delivery failure error (where x.x.x.x is your IP address) when sending email to Yahoo from a non-Yahoo Mail address.

This error indicates that Yahoo is receiving unusual traffic from your IP address.

This traffic can result from things outside of the sender's control. If you’re using a shared IP address—mail traffic from other domains could be negatively affecting your IP sending reputation. You’ll want to consider contacting your host provider to request using dedicated IP addresses to send your mail.

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. That still doesn't guarantee inbox delivery. You can fill out a new sender application if your email is being deferred by Yahoo.

Do all you can to make sure your emails reach your recipients. Reviewing our best practices FAQ and consider the following when you're sending email to Yahoo Mail addresses:

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