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Yahoo 421 4.7.0 [TSS04] — Temporarily Deferred Due to Volume or Complaints

Yahoo deferred mail from the sending IP because of unexpected volume or user complaints. TSS04 is Yahoo’s most common deferral tag.

Diagnose your exact bounce Email Infrastructure Digital Twin

What the bounce says

Exact wording as it appears in the rejection or NDR. Placeholders such as x.x.x.x and example.com stand for your own address and domain.

421 4.7.0 [TSS04] 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

421 4.7.1 [TSS07] Messages from x.x.x.x temporarily deferred - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

TSS04 is Yahoo’s standard deferral for senders it wants to slow down: unexpected volume or complaints from Yahoo users. It is temporary, but it persists as long as the traffic pattern that triggered it continues.

Reviewed 2026-09-08. Provider wording and requirements change; the provider reference below is authoritative.

Most common causes

  • A sudden increase in volume from the IP
  • User complaints about recent mail
  • Low engagement or sends to inactive addresses
  • New IP without history

What to verify next

  1. Retry with backoff; reduce sending rate to Yahoo
  2. Enrol in Yahoo’s complaint feedback loop and suppress complainers
  3. Remove inactive recipients
  4. Verify SPF, DKIM and DMARC alignment

Volume and complaints

A new IP sending at full volume, a large campaign after a quiet period, or a rising complaint rate all produce TSS04. Yahoo does not say which; the sending history usually does.

TSS07 is the neighbouring tag for deferrals without a stated reason, often reputation-related.

  • Sudden volume changes trigger it.
  • Complaints from Yahoo users sustain it.
  • TSS07 is the reason-less variant.

Reduce and retry

Let the queue back off, lower the hourly rate to Yahoo, and spread sends. Enrol in Yahoo’s complaint feedback loop to see who complains and suppress them. Remove inactive recipients, who complain most.

Authentication and identity

Confirm SPF, DKIM and DMARC alignment so Yahoo attributes reputation to your domain, and check PTR and HELO for the IP. Then use the sender hub form if deferrals persist after the pattern changes.

Best diagnostic path

Live analysis

Mail Failure Doctor

Classify the complete rejection and preserve provider-specific diagnostic context.

Open analysis → Live analysis

Email Infrastructure Digital Twin

Map the domain’s public mail infrastructure and provider relationships before remediation.

Open analysis →

Known limits

  • Yahoo does not publish thresholds.
  • Deferrals continue until sending behaviour changes.

Common questions

How long does TSS04 last?

Hours to days depending on volume and complaints. It lifts as the pattern normalizes.

Should I pause sending to Yahoo?

Reduce rather than pause; a pause followed by a burst repeats the trigger.

What is 4.16.55.1?

An internal Yahoo reference in the deferral text. It does not change the meaning.

Why the exact message matters

The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.

Provider reference

For the provider-defined meaning and current requirements, review Yahoo Sender Hub SMTP error codes.