421 4.7.0 [TSS04] Messages from IPADDRESS temporarily deferred due to unexpected volume or user complaints - 4.16.55.1

Products

• Messaging Gateway

Issue/Introduction

The emails are rejected by destination mail server and accumulate in the Messaging Gateway delivery queue with the following status: 

421 4.7.0 [TSS04] Messages from IPADDRESS temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

 

Cause

The destination mail server is rate limiting or otherwise deferring email delivery from the Messaging Gateway due to some email acceptance policy

Resolution

A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of the message. Message delivery can be delayed for multiple reasons including:

• Unusual traffic patterns from your sending server's IP address.
• The message that is attempted to send included characteristics of spam.
• Emails from the mail server have been generating complaints from Yahoo users.
• The Yahoo mail servers are busy when attempting a connection.
• Other suspicious behavior which leads Yahoo to enforce a dynamic deprioritization for the sender's server.

It is recommended to raise a case with 3rd party to obtain the support.

Additional Information

• https://senders.yahooinc.com/smtp-error-codes/
• https://senders.yahooinc.com/best-practices/