421 4.7.0 [TSS04] Yahoo: messages temporarily deferred
Quick answer
Yahoo is temporarily deferring mail from the sending IP. Yahoo documents its TS codes, such as [TS01] and [TSS04], as temporary deferrals caused by complaints, spam-like content, poor IP reputation or unusual traffic, and real [TSS04] bounces cite unexpected volume or user complaints. Nothing is wrong with the recipients.
Recommended action: Retry later
Common causes
- A spike in volume from the IP or domain.
- Complaints from Yahoo users marking recent mail as spam.
- Sending to many inactive or invalid Yahoo and AOL addresses.
What to do
Let the mail retry and reduce volume to Yahoo. Remove unengaged recipients, verify the list, and make sure one-click unsubscribe works. Deferrals ease once complaint and volume patterns improve.
Provider note: RocketVerifier reads every Yahoo [TS..], [TSS..] and [BL..] code as a block on the sender. During verification the address comes back unknown with the blocked sub-status and is never charged.
How to read 421 4.7.0 [TSS04]
421 is the basic SMTP reply defined in RFC 5321. 4.7.0 is the enhanced status code from RFC 3463, which reads as class, subject and detail:
- Class
- 4Temporary failure
- Subject
- .7Security or policy
- Detail
- .0Yahoo: messages temporarily deferred
Further reading
FAQ
Frequently asked questions
Will Yahoo retry or should my server?
Your server retries. A 421 is a temporary failure, so a standards-compliant mail server queues the message and tries again later. Keep volume to Yahoo low while you fix the cause.
Stop hard bounces before they start
Upload a list or call the API, and send only to addresses that can receive mail. Unknown results are always free.
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