Yahoo 421
Temporary deferral (TS errors)
Yahoo 421 means: temporary deferral (TS errors). It is a temporary failure, so your server retries it. Let the queue retry and slow down to Yahoo and AOL domains. Check complaint rates and stop mailing addresses that never engage. If the same TS deferral lasts for days, file Yahoo's Sender Support Request with the full error text.
Yahoo’s exact wording
Quoted from Yahoo Sender Hub: SMTP Error Codes, checked 2026-11-05. Placeholders are Yahoo’s own.
Provider's description
This is a temporary error and your mail server may automatically re-try sending the email at a later time.
Provider's description
The IP (x.x.x.x or its subnet, i.e., .255 ) has a poor reputation.
Provider's description
If you're seeing the same error consistently over an extended period of time, we encourage you to submit a Sender Support Request with specific details of the error and diagnostic codes you see in your logs.
Provider's description
This error indicates that the Yahoo mail servers were busy and temporarily unable to process your transaction at the time of connection.
Provider's description
Such issues are generally brief, and normal connectivity is usually restored after a short period of time.
Possible cause
We've observed unusual traffic patterns from your sending server's IP address.
Possible cause
The message you attempted to send included characteristics of spam.
Possible cause
Emails from your mail server have been generating complaints from Yahoo users.
Possible cause
The Yahoo mail servers are busy when attempting a connection.
Possible cause
Other suspicious behavior which leads Yahoo to enforce a dynamic deprioritization for your sender's server.
Possible cause
Your name server was temporarily unavailable.
Possible cause
Authentication results for your domain could not be determined due to a temporary error.
| Who sends it | Yahoo and AOL |
|---|---|
| Basic code | 421: <domain> Service not available, closing transmission channel (RFC) |
| Enhanced code | None given |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | Partly |
What the sender should do
Let the queue retry and slow down to Yahoo and AOL domains. Check complaint rates and stop mailing addresses that never engage. If the same TS deferral lasts for days, file Yahoo's Sender Support Request with the full error text.
Would verifying the list have prevented Yahoo 421?
Partly
The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Common questions
- What does Yahoo 421 mean?
- Yahoo 421 means: temporary deferral (TS errors). It is a temporary failure, so your server retries it. Let the queue retry and slow down to Yahoo and AOL domains. Check complaint rates and stop mailing addresses that never engage. If the same TS deferral lasts for days, file Yahoo's Sender Support Request with the full error text.
- Should I retry after Yahoo 421?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Yahoo 421?
- Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Sources
- Yahoo Sender Hub: SMTP Error Codes (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05