Yahoo [TSS04]: mail temporarily deferred
421 4.7.0 [TSS04]Temporarily deferred for unexpected volume or user complaintsTemporary: the sender retriesFrom Yahoo Mail and AOL
421 4.7.0 [TSS04] Messages from 203.0.113.70 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1
What it means
TSS04 is the most common deferral Yahoo hands out. Yahoo’s filters have seen something about your recent traffic they do not like (a jump in volume, a rise in “this is spam” clicks, or mail to many addresses that no longer exist) and are slowing acceptance from your IP. The response usually reads like the example, with your IP and an internal reference after the text.
The 421 reply tells your server to keep the message queued and retry. Most TSS04 episodes clear within hours if you stop pushing. If the behaviour that triggered it continues, Yahoo can escalate to TSS09, which no amount of retrying will get past.
Likely causes
- A campaign to a large or old segment of Yahoo and AOL addresses sent faster than your usual pace.
- Complaint rates above Yahoo’s expectations; Yahoo asks bulk senders to keep the spam rate under 0.3 percent.
- Many recipients on the list are invalid, which signals poor list hygiene.
- A new IP address that has not been warmed up sending at full volume.
How to fix it
- Let retries happen at a low rate. Reduce concurrent connections to Yahoo MX hosts rather than retrying harder.
- Pause marketing sends to Yahoo and AOL addresses until deferrals fade; keep transactional mail flowing first.
- Sign up for Yahoo’s complaint feedback loop through Yahoo Sender Hub (senders.yahooinc.com) and suppress everyone who complains.
- Remove addresses that hard bounced and recipients with no opens or clicks in many months.
- When volume resumes, increase it in steps over several days.
Checks that apply
- Blacklist Check Checks whether the IP Yahoo named is also listed on public DNSBLs, which points to a real spam problem.
- Google/Yahoo Compliance Covers the shared Google and Yahoo bulk-sender rules, including one-click unsubscribe and DMARC.
- MailFlow Monitor Sends test mail on a schedule so you can see when Yahoo stops deferring.
FAQ
- How long does a TSS04 deferral last?
- Usually hours, sometimes a day or two. It depends on how quickly the complaint and volume pattern returns to normal.
- Should I contact Yahoo support?
- Fix the traffic first. Yahoo Sender Hub has a support form for senders, but it rarely speeds up a deferral that the traffic itself is causing.