Troubleshooting Tip: How to resolve ‘421 4.7.0 [TSS05] Messages from x.x.x.x temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes’

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Description	This article describes How to resolve ‘421 4.7.0 [TSS05] Messages from x.x.x.x105 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes’
Scope	FortiMail.
Solution	Sample log will be seen in FortiMail : '421 4.7.0 [TSS05] Messages from x.x.x.x temporarily deferred due to unexpected volume or user complaints - 4.16.55.1', it indicates that the email messages are being temporarily blocked or deferred by Yahoo due to one of the following reasons: An unexpected volume of emails is being sent from the mail server. User complaints, such as spam reports or misuse of the email service. Reputation Issues: The mail server’s IP address may have a poor reputation due to previous abuse or poor email-sending practices Yahoo Mail implements these blocks to maintain a high standard of email deliverability and protect its users from spam and abuse. Yahoo SMTP Error codes: https://senders.yahooinc.com/smtp-error-codes/ The workaround that will be helpful for this issue : Contact Yahoo Postmaster to clarify the issue or request to delist the IP. Verify the source of the problem. Reduce email sending volume.