SMTP code guide

SMTP 550 5.7.1: provider meanings and fixes

SMTP 550 5.7.1 can mean policy rejection, authorization failure, blocked content, or rejected sender. Compare Gmail, Microsoft, and Apple handling before retrying or suppressing.

Quick summary

Primary code
550 5.7.1
Provider coverage
Gmail, Microsoft, Apple
Primary action
Fix sender setup

How to handle 550 5.7.1

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 550 5.7.1

Reputation or policy

Fix sender setup

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

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: high. Recipient validity: unknown.

Match this reply

SMTP stage
End of DATA
Endpoint
Inbound MX
likely unsolicitedUnsolicitedMessageError

Start here

  1. 1

    Pause or slow sending to that provider while you review complaint rate, bounce rate, and recent volume changes.

  2. 2

    Check SPF, DKIM, DMARC alignment and make sure the message is expected by recipients.

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

  • Treat this as a sender setup incident, not a bad recipient.
  • Hold similar mail for the same sender domain or provider until the fix is verified.
  • Retry after a fresh single-recipient test succeeds.

Avoid

  • Do not retry unchanged in a loop.
  • Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
  • Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
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
likely unsolicited
Evidence
official+observed
support.google.com source 1

Gmail 550 5.7.1

Message format

Fix sender setup

The provider rejected the message structure. Check headers, MIME formatting, line endings, and required message fields before sending again.

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: high. Recipient validity: unknown.

Match this reply

SMTP stage
End of DATA
Endpoint
Inbound MX
duplicate headersFrom: header is missingmultiple FromMessage-ID

Start here

  1. 1

    Validate the message headers and MIME structure before sending again.

  2. 2

    Check for malformed From, Sender, Date, Message-ID, or line-ending issues.

Confirm the cause

  1. Validate headers, MIME boundaries, line endings, Date, From, Sender, and Message-ID.
  2. Send a minimal message from the same SMTP client to isolate formatter issues from content issues.
  3. Fix the message generator before retrying the original message unchanged.

App handling

  • Treat this as a sender setup incident, not a bad recipient.
  • Hold similar mail for the same sender domain or provider until the fix is verified.
  • Retry after a fresh single-recipient test succeeds.

Avoid

  • Do not retry unchanged in a loop.
  • Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
  • Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
Common causes
  • Required headers are missing, malformed, duplicated, or contradictory.
  • MIME boundaries, line endings, or message structure are invalid.
  • The message was generated by a client that does not follow SMTP or RFC 5322 expectations.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
Specific cause
malformed rfc5322 headers
Evidence
official
support.google.com source 1

Microsoft mail servers 550 5.7.1

Policy rejection

Fix sender setup

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

Broad Microsoft policy bucket.

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: medium. Recipient validity: unknown.

Match this reply

SMTP stage
Any SMTP stage
Endpoint
Inbound MX
Delivery not authorizedpolicy

Start here

  1. 1

    Review the provider diagnostic text for the specific policy that was triggered.

  2. 2

    Do not keep retrying unchanged if the reply is permanent. Fix the sender, content, or recipient policy first.

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

  • Treat this as a sender setup incident, not a bad recipient.
  • Hold similar mail for the same sender domain or provider until the fix is verified.
  • Retry after a fresh single-recipient test succeeds.

Avoid

  • Do not retry unchanged in a loop.
  • Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
  • Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
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 domaindecisiontimestamp
Classifier evidence
Specific cause
delivery not authorized
Evidence
official
learn.microsoft.com source 1

Microsoft mail servers 550 5.7.1

SMTP authentication

Fix sender setup

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

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: high. Recipient validity: unknown.

Match this reply

SMTP stage
MAIL FROM
Endpoint
SMTP submission
Client was not authenticated

Start here

  1. 1

    Check SMTP credentials, OAuth token state, and whether SMTP AUTH is enabled for the mailbox or tenant.

  2. 2

    Refresh credentials or reconnect the sending account before retrying.

Confirm the cause

  1. Confirm the SMTP credentials, OAuth scopes, and token expiry for the sending account.
  2. Check whether SMTP AUTH is enabled for the mailbox or tenant.
  3. Reconnect the account or refresh credentials, then test with one message before resuming volume.

App handling

  • Treat this as a sender setup incident, not a bad recipient.
  • Hold similar mail for the same sender domain or provider until the fix is verified.
  • Retry after a fresh single-recipient test succeeds.

Avoid

  • Do not retry unchanged in a loop.
  • Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
  • Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
Common causes
  • SMTP credentials, OAuth permissions, or access tokens are invalid or expired.
  • SMTP AUTH is disabled for the mailbox, tenant, or account type.
  • The SMTP client is authenticating against the wrong endpoint or account.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
Specific cause
client not authenticated
Evidence
official
learn.microsoft.com source 1

Apple 550 5.7.1

Policy rejection

Fix sender setup

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

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: medium. Recipient validity: unknown.

Match this reply

SMTP stage
End of DATA
Endpoint
Inbound MX
Delivery not authorized

Start here

  1. 1

    Review the provider diagnostic text for the specific policy that was triggered.

  2. 2

    Do not keep retrying unchanged if the reply is permanent. Fix the sender, content, or recipient policy first.

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

  • Treat this as a sender setup incident, not a bad recipient.
  • Hold similar mail for the same sender domain or provider until the fix is verified.
  • Retry after a fresh single-recipient test succeeds.

Avoid

  • Do not retry unchanged in a loop.
  • Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
  • Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
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 domaindecisiontimestamp
Classifier evidence
Specific cause
delivery not authorized
Evidence
observed
support.apple.com source 1

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GmailMicrosoftApple