SMTP code guide

SMTP 421 4.7.0: provider meanings and fixes

SMTP 421 4.7.0 can mean temporary deferral caused by rate limits, sender reputation, or policy checks. Compare Gmail and Yahoo/AOL handling before retrying or suppressing.

Quick summary

Primary code
421 4.7.0
Provider coverage
Gmail, Yahoo/AOL
Primary action
Retry later

How to handle 421 4.7.0

Start by matching the provider wording, then use the matching section below. The same SMTP code can call for different actions depending on whether it points to recipient validity, routing, sender setup, or policy.

Gmail 421 4.7.0

Temporary policy or reputation

Retry later

Temporary policy or reputation means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.

Broad Gmail bucket. Diagnostic text is required for a narrower cause.

Decision

A retry can work, especially after a short wait.

Confidence: medium. Recipient validity: unknown.

Match this reply

SMTP stage
Any SMTP stage
Endpoint
Inbound MX
temporary / ilytry again laterratereputationsuspicious

Start here

  1. 1

    Read the full SMTP diagnostic text, not only the numeric reply code.

  2. 2

    Identify the provider, SMTP stage, and any provider token before deciding whether to retry or suppress.

Confirm the cause

  1. Review recent sending volume, bounce rate, complaint rate, and list hygiene for this provider.
  2. Confirm SPF, DKIM, and DMARC alignment so reputation problems are not made worse by authentication failures.
  3. Resume with lower provider-specific volume only after deferrals stop or the provider accepts test mail.

App handling

  • Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
  • Slow or pause provider-specific sending if the same deferral repeats across many recipients.
  • Escalate to reputation, authentication, or volume review when retries keep failing.

Avoid

  • Do not suppress the recipient from a temporary response.
  • Do not keep the same retry pace if the deferral repeats across many recipients.
  • Do not ignore sender reputation or authentication if this keeps happening.
Common causes
  • Volume increased faster than the provider trusts for this sender.
  • Recent bounces, complaints, or low engagement damaged sender reputation.
  • The provider applied a traffic, content, or sender policy to protect recipients.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampprovider retry countsend volume windowbounce rate
Classifier evidence
Specific cause
gmail generic 4 7 0
Evidence
official
support.google.com source 1

Yahoo/AOL 421 4.7.0

Temporary reputation or volume throttle

Retry later

Temporary reputation or volume throttle means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.

Observed on Yahoo and AOL-hosted Yahoo infrastructure. Do not reduce to 'complaints only'.

Decision

A retry can work, especially after a short wait.

Confidence: high. Recipient validity: unknown.

Match this reply

SMTP stage
Any SMTP stage
Endpoint
Inbound MX
[TSS04]temporarily deferred

Start here

  1. 1

    Read the full SMTP diagnostic text, not only the numeric reply code.

  2. 2

    Identify the provider, SMTP stage, and any provider token before deciding whether to retry or suppress.

Confirm the cause

  1. Review recent sending volume, bounce rate, complaint rate, and list hygiene for this provider.
  2. Confirm SPF, DKIM, and DMARC alignment so reputation problems are not made worse by authentication failures.
  3. Resume with lower provider-specific volume only after deferrals stop or the provider accepts test mail.

App handling

  • Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
  • Slow or pause provider-specific sending if the same deferral repeats across many recipients.
  • Escalate to reputation, authentication, or volume review when retries keep failing.

Avoid

  • Do not suppress the recipient from a temporary response.
  • Do not keep the same retry pace if the deferral repeats across many recipients.
  • Do not ignore sender reputation or authentication if this keeps happening.
Common causes
  • Volume increased faster than the provider trusts for this sender.
  • Recent bounces, complaints, or low engagement damaged sender reputation.
  • The provider applied a traffic, content, or sender policy to protect recipients.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampprovider retry countsend volume windowbounce rate
Classifier evidence
Specific cause
yahoo tss04
Evidence
official+observed
senders.yahooinc.com source 1

Yahoo/AOL 421 4.7.0

Temporary reputation or volume throttle

Retry later

Temporary reputation or volume throttle means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.

Decision

A retry can work, especially after a short wait.

Confidence: medium. Recipient validity: unknown.

Match this reply

SMTP stage
Any SMTP stage
Endpoint
Inbound MX
[TSS05]temporarily deferred

Start here

  1. 1

    Read the full SMTP diagnostic text, not only the numeric reply code.

  2. 2

    Identify the provider, SMTP stage, and any provider token before deciding whether to retry or suppress.

Confirm the cause

  1. Review recent sending volume, bounce rate, complaint rate, and list hygiene for this provider.
  2. Confirm SPF, DKIM, and DMARC alignment so reputation problems are not made worse by authentication failures.
  3. Resume with lower provider-specific volume only after deferrals stop or the provider accepts test mail.

App handling

  • Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
  • Slow or pause provider-specific sending if the same deferral repeats across many recipients.
  • Escalate to reputation, authentication, or volume review when retries keep failing.

Avoid

  • Do not suppress the recipient from a temporary response.
  • Do not keep the same retry pace if the deferral repeats across many recipients.
  • Do not ignore sender reputation or authentication if this keeps happening.
Common causes
  • Volume increased faster than the provider trusts for this sender.
  • Recent bounces, complaints, or low engagement damaged sender reputation.
  • The provider applied a traffic, content, or sender policy to protect recipients.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampprovider retry countsend volume windowbounce rate
Classifier evidence
Specific cause
yahoo tss05
Evidence
observed
senders.yahooinc.com source 1

Yahoo/AOL 421 4.7.0

Temporary policy or reputation

Retry later

Temporary policy or reputation means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.

Decision

A retry can work, especially after a short wait.

Confidence: medium. Recipient validity: unknown.

Match this reply

SMTP stage
Any SMTP stage
Endpoint
Inbound MX
[TS01 / TSS00]temporarily deferred

Start here

  1. 1

    Read the full SMTP diagnostic text, not only the numeric reply code.

  2. 2

    Identify the provider, SMTP stage, and any provider token before deciding whether to retry or suppress.

Confirm the cause

  1. Review recent sending volume, bounce rate, complaint rate, and list hygiene for this provider.
  2. Confirm SPF, DKIM, and DMARC alignment so reputation problems are not made worse by authentication failures.
  3. Resume with lower provider-specific volume only after deferrals stop or the provider accepts test mail.

App handling

  • Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
  • Slow or pause provider-specific sending if the same deferral repeats across many recipients.
  • Escalate to reputation, authentication, or volume review when retries keep failing.

Avoid

  • Do not suppress the recipient from a temporary response.
  • Do not keep the same retry pace if the deferral repeats across many recipients.
  • Do not ignore sender reputation or authentication if this keeps happening.
Common causes
  • Volume increased faster than the provider trusts for this sender.
  • Recent bounces, complaints, or low engagement damaged sender reputation.
  • The provider applied a traffic, content, or sender policy to protect recipients.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampprovider retry countsend volume windowbounce rate
Classifier evidence
Specific cause
yahoo general deferral
Evidence
observed
senders.yahooinc.com source 1

Not sure this is the right match?

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GmailYahoo/AOL