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421 4.7.0: Message temporarily deferred

The receiving server is temporarily refusing your mail, usually over IP or domain reputation, a missing PTR record, or volume.

Bounce type
Soft bounce (temporary)
What to do
Slow down, then retry
Returned by
Gmail / Google Workspace; Yahoo / AOL; Any mail server

What 421 4.7.0 means

421 is a temporary 'service not available' reply and 4.7.0 is the catch-all security or policy status. Together they say: we're not taking this message right now, for a policy reason, try again later. The server usually closes the connection after sending it.

4.7.0 is generic, so the text after the code is what tells you why. Gmail uses it for at least eight different situations, from an expired connection to 'very low reputation of the sending domain'. Yahoo's well-known [TSS04] deferral also uses 421 4.7.0.

What the bounce looks like

The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.

Gmail
421 4.7.0 This message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked.
Gmail
421 4.7.0 The IP address sending this message does not have a PTR record, or the corresponding forward DNS entry does not point to the sending IP. To protect our users from spam, email sent from your IP address has been temporarily rate limited.
Gmail
421 4.7.0 Connection expired, try reconnecting.

Harmless. The SMTP session timed out; the retry will go through.

Yahoo
421 4.7.0 [TSS04] Messages from [IP] temporarily deferred due to unexpected volume or user complaints

Yahoo's TS* codes are temporary deferrals; Yahoo says your server may retry later.

Why cold emailers see 421 4.7.0

  • Low IP or domain reputation: recent spam complaints, a burst of volume from a new domain, or a shared IP that other senders have damaged.
  • A sending IP without a PTR (reverse DNS) record, or one whose forward lookup doesn't match. Common on self-hosted or cheap SMTP servers.
  • Content or links Gmail considers suspicious, including link shorteners and tracking domains with a poor history.
  • Yahoo [TSS04]: unexpected volume from your IP or complaints from Yahoo users about earlier mail.
  • Benign cases: the connection expired, or the server was busy. These clear on their own.

How to fix 421 4.7.0

  1. Read the full text after the code. 'Connection expired' or 'try again later' needs nothing; 'low reputation' or 'complaints' needs action.
  2. Let your sending server retry on its normal backoff. Don't hammer the server with immediate resends; that extends the deferral.
  3. Cut daily volume from the affected inboxes and domains by half or more until deferrals stop, then ramp up slowly.
  4. Check that SPF, DKIM and DMARC pass for the sending domain and that your sending IP has a PTR record that resolves back to it.
  5. Remove link shorteners and recently registered tracking domains from your copy. Plain-text first touches with no links get deferred less.
  6. Check Google Postmaster Tools for spam rate and domain reputation. Google asks senders to stay below a 0.1% spam rate and never reach 0.3%.

Will it retry on its own?

Don't suppress the recipient. The address is probably fine; the receiver doesn't trust your traffic yet. If the same deferral keeps coming back for days, the message eventually expires and bounces, and continued pressure can turn it into a 5xx block.

Reading the code

The basic code 421 means "Service not available, closing connection (temporary)" under RFC 5321. In 4.7.0, the 4 is the class (persistent transient failure) and the 7 is the subject (security or policy). The detail number 0 is registered in RFC 3463 / the IANA registry as "Other or undefined security status".

Class 4
Persistent transient failure
A temporary condition delayed delivery. The message itself is valid, and sending again later may succeed.
Subject 7
Security or policy
A security or policy decision: spam filtering, authentication, blocklists, permissions.
Detail 4.7.0
Other or undefined security status

Check the rest of your setup

Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.

Got a different bounce? Paste it into the SMTP error code lookup to decode it.

FAQ

Is 421 4.7.0 a hard bounce or a soft bounce?

421 4.7.0 is a temporary failure (a soft bounce). Your sending server should retry automatically, and the message is often delivered on a later attempt. If it keeps failing for days, the server gives up and reports a final bounce.

Should I remove the contact after a 421 4.7.0 bounce?

No. Don't suppress the recipient. The address is probably fine; the receiver doesn't trust your traffic yet. If the same deferral keeps coming back for days, the message eventually expires and bounces, and continued pressure can turn it into a 5xx block.

What does 4.7.0 mean in the error code?

In 4.7.0, the 4 is the class (persistent transient failure) and the 7 is the subject (security or policy). The detail number 0 is registered in RFC 3463 / the IANA registry as "Other or undefined security status".

Related SMTP codes

See all SMTP error codes

Sources

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