Yahoo 421 4.7.0 [TSS04]: “temporarily deferred due to unexpected volume or user complaints”

Short answer: Yahoo (and AOL) are temporarily deferring mail from your IP because of complaints, spam-like content, poor IP reputation or unusual volume. Your server retries automatically. Fix the cause and keep volume steady, and if it persists, send a Sender Support Request to Yahoo with the full error.

The bounce looks like this
421 4.7.0 [TSS04] Messages from 203.0.113.10 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

What it means

Yahoo's TS codes are temporary deferrals. Yahoo lists the usual causes as complaints, spam-like content, poor IP reputation and unusual traffic. It is common after a big campaign, on a new IP, or when many recipients mark the mail as spam. Mail is retried, so most of it still arrives late. If it keeps happening, it means Yahoo trusts your traffic less.

How to fix it

  1. Meet Yahoo's sender requirements first: SPF or DKIM for everyone, and SPF, DKIM and DMARC for bulk senders. Check your domain below.
  2. Make unsubscribing easy (one-click List-Unsubscribe for marketing mail) and keep the spam complaint rate under 0.3%.
  3. Send to people who asked for your mail: remove unengaged and bouncing addresses and never send to bought lists.
  4. Spread large sends over hours instead of minutes, especially from a new IP.
  5. Check outgoing queues for spam from a compromised account or form.
  6. If the deferral continues, submit a Sender Support Request to Yahoo with the error text and the diagnostic codes from your logs.

Check your domain now

After the fix, send a test email to see SPF, DKIM and DMARC pass on a real message, then keep an eye on it: free DMARC monitoring shows every service that sends mail as your domain and which ones fail, in plain English.

Rather not touch DNS? I set up SPF, DKIM and DMARC without breaking your other senders, with a before/after report.

Get it fixed on Fiverr

Prefer to do it yourself? Get the step-by-step Fix Kit ($15) – 28-page guide, checklist, 20+ providers.

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