421 Yahoo bounce: temporary block, retry
What 421 means
Yahoo is deferring your message for now; the rejection is temporary and retrying later is allowed.
Message text
A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of your message.
Common causes of 421
- Unusual traffic from the sending IP
- Spam-like content or Yahoo complaints
- Busy receiving servers
- Authentication lookups that failed temporarily
How to fix 421
- Let the sending queue retry later; the message normally delivers without action.
- Check the sending IP with blocklist check and review complaint rates and content.
- Confirm SPF and DKIM records with SPF checker and DKIM checker.
Check your domain
Check the SPF record of your sending domainRuns in your browser; nothing is uploaded.
Get your domain's deliverability score
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 421
The first part, 421, is the SMTP reply code. Service not available, closing transmission channel: the server is shutting down or refusing the connection for now. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
This reply carries no enhanced status code. The three-digit reply code is the only machine-readable signal, so read the text that follows it to learn the reason.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: senders.yahooinc.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
FAQ
What does 421 mean?
Yahoo is deferring your message for now; the rejection is temporary and retrying later is allowed. This is a temporary failure.
How do I fix 421?
Let the sending queue retry later; the message normally delivers without action. Check the sending IP with blocklist check and review complaint rates and content. Confirm SPF and DKIM records with SPF checker and DKIM checker.
Is 421 permanent, and should I retry?
421 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.