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Yahoo 421 4.7.0 [TSS04]: messages temporarily deferred

How-to guide
For:
Publishers and marketers, Workspace admins
Needs permission:
providers:view, providers:managesensitive, campaigns:sendsensitive
In the app:
Sending providers, then Events & sync

Yahoo TSS04 is a temporary deferral: Yahoo’s servers answer 421 4.7.0 with the tag [TSS04] when they want less mail from you for now, most often because of complaints from Yahoo users or a sudden rise in volume. Your sending provider retries later. This page explains what Yahoo looks at and how to bring the deferrals down.

What Yahoo means

Section titled “What Yahoo means”

Yahoo’s servers reply with a line such as:

SMTP reply
421 4.7.0 [TSS04] Messages from 203.0.113.25 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

Yahoo’s Sender Hub describes these “TS” errors as message temporarily deferred:

This is a temporary error and your mail server may automatically re-try sending the email at a later time. Your message may have been deferred due to one or more of the following:

  • Emails from your mail server are generating substantial complaints from Yahoo users.
  • The message contained objectionable content or exhibited characteristics indicative of spam.
  • The IP (x.x.x.x or its subnet, i.e., .255 ) has a poor reputation.
  • We are seeing unusual traffic patterns from your mail servers.

Source: SMTP Error Codes, Yahoo Sender Hub. Checked October 2026.

Why it happens

Section titled “Why it happens”
  • Yahoo users mark your mail as spam. Yahoo asks every sender to keep the spam rate below 0.3% (Sender Requirements & Recommendations, checked October 2026).
  • Volume jumps, for example the first large campaign from a new IP address or sending provider.
  • The sending IP address, or its range, has a poor reputation.
  • The content looks like spam.

How ReputeCast classifies it

Section titled “How ReputeCast classifies it”
  • While the sending provider retries, each refused attempt is a deferral: a Deferred event in the event log. Deferrals don’t appear in the campaign report’s bounces table.
  • Deferrals count toward the Yahoo mailbox provider’s deferral rate during a send. Above 5.0% (fixed; it counts after 500 messages to Yahoo and at least 5 deferrals, with about 90% confidence), ReputeCast auto-pauses the Yahoo share of the campaign, or the whole campaign when the pause scope is set to the campaign.
  • If the sending provider stops retrying, the bounce class is reputation, from the TSS04 tag, and the report row is Blocked · policy or reputation. A bounce with a 4xx reply is recorded as a soft bounce and doesn’t suppress the address, unless the sending provider itself calls it permanent (see when ReputeCast suppresses an address).
  • When a sending provider passes only 421 4.7.0 without the tag, the class is temporary (Soft · deferral expired).

Before you begin

Section titled “Before you begin”
  • providers:view to read the event log.
  • providers:manage to change the throttles in Settings, then Providers.
  • campaigns:send to resume an auto-paused send.

Bring the deferrals down

Section titled “Bring the deferrals down”
  1. Confirm the code. Open Sending providers, then Events & sync, set Event type to Deferred and search for a Yahoo recipient. Original code · diagnostic shows the [TSS04] tag and the IP address Yahoo deferred.
  2. Slow down to Yahoo. By default ReputeCast throttles Yahoo to 4 messages per second on every route; lower it in Settings, then Providers, then Throttles per mailbox provider. During an auto-paused send, the live monitor recommends a lower rate for the Yahoo share; resume with that change.
  3. Bring complaints down. Send only to contacts who asked for your mail, exclude those who stopped opening (keep your list healthy), and keep one-click unsubscribe working so readers leave instead of reporting spam.
  4. If the IP address or sending provider is new, follow a warm-up plan.
  5. If the deferrals go on for a long time, Yahoo’s Sender Hub suggests a Sender Support Request with the error and diagnostic codes from your logs. The event log’s CSV export has them.

Check that it worked

Section titled “Check that it worked”
  • The live monitor’s Yahoo deferral rate stays low during the next send.
  • The event log shows delivered events for Yahoo recipients instead of new [TSS04] deferrals.

Troubleshooting

Section titled “Troubleshooting”
  • The send keeps pausing on Yahoo. Resume the Yahoo share at the lower rate the live monitor recommends, and lower the Yahoo throttle for future sends.
  • Yahoo complaints are high. Read bounces, complaints and provider limits; by default a running send auto-pauses when a mailbox provider’s complaint rate passes 0.10%.
  • You send from a shared IP address. Yahoo’s Sender Hub notes that on a shared IP address, mail from other domains can hurt your IP reputation, and suggests asking your provider for dedicated IP addresses. A new dedicated IP address needs a warm-up plan.
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