Yahoo TSS04
Yahoo temporary deferral
TSS04 is a Yahoo temporary deferral, sent with a 421 4.7.0 reply. Yahoo puts it in its "TS" family of deferrals and lists causes such as user complaints, spam-like content, poor IP reputation and unusual traffic. Mail retries automatically, but it means Yahoo is unhappy with your recent sending.
Should you remove the address?
Keep the address; fix the sending side
Keep it. TSS04 is about your sending, not about any one address.
Soft bounce. Your sending server retries on its own. Investigate only if it keeps failing.
What causes it
- User complaints about recent mail from your IP or domain.
- Content Yahoo considers spam-like.
- Poor reputation for the IP or its subnet.
- Unusual traffic patterns, such as a sudden jump in volume.
How to fix it
In order — try the first one first.
- Let the queue retry, and lower the sending rate to Yahoo domains.
- Look at complaint rates. Yahoo offers a complaint feedback loop for senders.
- Remove unengaged and unverified addresses before the next send to Yahoo.
- If it lasts a long time, Yahoo asks senders to file a Sender Support Request with the error details.
Who sends it
Yahoo Mail, which also hosts AOL and other Yahoo-operated domains.
Yahoo
Yahoo's page groups these as "Message temporarily deferred (eg: TS* Errors)" and lists complaints, objectionable content, poor IP or subnet reputation and unusual traffic patterns. It does not publish the full text of each TS code.
Common questions
- What does SMTP Yahoo TSS04 mean?
- TSS04 is a Yahoo temporary deferral, sent with a 421 4.7.0 reply. Yahoo puts it in its "TS" family of deferrals and lists causes such as user complaints, spam-like content, poor IP reputation and unusual traffic. Mail retries automatically, but it means Yahoo is unhappy with your recent sending.
- Should I retry a Yahoo TSS04 error?
- Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
- How do I fix SMTP Yahoo TSS04?
- Let the queue retry, and lower the sending rate to Yahoo domains.
- Is Yahoo TSS04 a hard bounce or a soft bounce?
- Yahoo TSS04 is a soft bounce. It is temporary, and your sending server retries it automatically. Your sending server retries on its own. Investigate only if it keeps failing.
- Should I remove the address after a Yahoo TSS04 bounce?
- Keep it. TSS04 is about your sending, not about any one address.
- Will verifying my list prevent Yahoo TSS04?
- Only partly. Temporary deferrals are not about the address, but providers defer senders whose mail looks risky, and a high rate of hard bounces is one of the signals they use.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
One plan, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.
Other 421 replies
Sources
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22