SMTP code guide

SMTP 550 5.1.1: provider meanings and fixes

SMTP 550 5.1.1 can mean invalid recipient, unavailable mailbox, or provider-specific authorization issue. Compare Gmail, Microsoft, and Apple handling before retrying or suppressing.

Quick summary

Primary code
550 5.1.1
Provider coverage
Gmail, Microsoft, Apple
Primary action
Suppress recipient

How to handle 550 5.1.1

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 5.1.1

Recipient issue

Suppress recipient

The provider is rejecting the mailbox or account. Only treat it as invalid when the provider-specific text, endpoint, and stage all point to recipient validation.

Decision

Suppress this recipient. A retry is unlikely to help.

Confidence: high. Recipient validity: invalid.

Match this reply

SMTP stage
RCPT TO recipient check
Endpoint
Inbound MX
does not existNoSuchUseruser unknown

Start here

  1. 1

    Confirm the recipient address and domain are spelled correctly.

  2. 2

    Suppress the recipient only when the provider-specific evidence says the mailbox is invalid.

Confirm the cause

  1. Validate the recipient address and domain before suppressing it.
  2. Check whether the mailbox, alias, group, or contact exists and can receive external mail.
  3. Keep the full SMTP reply with the contact record so support can distinguish invalid addresses from policy blocks.

App handling

  • Classify as a hard bounce only when this provider-specific fingerprint matches.
  • Stop retries for the bounced recipient and keep the raw SMTP reply for audit and support.
  • Do not suppress other recipients in the same send unless they return their own matching bounce.

Avoid

  • Do not suppress from the numeric code alone. Confirm the provider, SMTP stage, and diagnostic text first.
  • Do not keep retrying once the provider-specific evidence says the mailbox is invalid.
  • Do not apply this suppression decision to the same code from another provider without matching that provider's text.
Common causes
  • The address is misspelled, deleted, disabled, or not mail-enabled.
  • The recipient domain accepts mail, but the specific mailbox cannot receive this message.
  • An alias, distribution group, or forwarded address no longer maps to a valid mailbox.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampenvelope recipientcontact id
Classifier evidence
Specific cause
invalid recipient
Evidence
official+observed
support.google.com source 1

Microsoft mail servers 550 5.1.1

Recipient issue

Suppress recipient

The provider is rejecting the mailbox or account. Only treat it as invalid when the provider-specific text, endpoint, and stage all point to recipient validation.

Decision

Suppress this recipient. A retry is unlikely to help.

Confidence: high. Recipient validity: invalid.

Match this reply

SMTP stage
RCPT TO recipient check
Endpoint
Inbound MX
Bad destination mailboxuser unknowndoes not exist

Start here

  1. 1

    Confirm the recipient address and domain are spelled correctly.

  2. 2

    Suppress the recipient only when the provider-specific evidence says the mailbox is invalid.

Confirm the cause

  1. Validate the recipient address and domain before suppressing it.
  2. Check whether the mailbox, alias, group, or contact exists and can receive external mail.
  3. Keep the full SMTP reply with the contact record so support can distinguish invalid addresses from policy blocks.

App handling

  • Classify as a hard bounce only when this provider-specific fingerprint matches.
  • Stop retries for the bounced recipient and keep the raw SMTP reply for audit and support.
  • Do not suppress other recipients in the same send unless they return their own matching bounce.

Avoid

  • Do not suppress from the numeric code alone. Confirm the provider, SMTP stage, and diagnostic text first.
  • Do not keep retrying once the provider-specific evidence says the mailbox is invalid.
  • Do not apply this suppression decision to the same code from another provider without matching that provider's text.
Common causes
  • The address is misspelled, deleted, disabled, or not mail-enabled.
  • The recipient domain accepts mail, but the specific mailbox cannot receive this message.
  • An alias, distribution group, or forwarded address no longer maps to a valid mailbox.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampenvelope recipientcontact id
Classifier evidence
Specific cause
invalid recipient
Evidence
official
learn.microsoft.com source 1

Apple 550 5.1.1

Recipient issue

Suppress recipient

The provider is rejecting the mailbox or account. Only treat it as invalid when the provider-specific text, endpoint, and stage all point to recipient validation.

Only safe when endpoint/stage/text clearly indicate inbound MX recipient validation.

Decision

Suppress this recipient. A retry is unlikely to help.

Confidence: medium. Recipient validity: invalid.

Match this reply

SMTP stage
RCPT TO recipient check
Endpoint
Inbound MX
user does not existuser does not existsunknown user

Start here

  1. 1

    Confirm the recipient address and domain are spelled correctly.

  2. 2

    Suppress the recipient only when the provider-specific evidence says the mailbox is invalid.

Confirm the cause

  1. Validate the recipient address and domain before suppressing it.
  2. Check whether the mailbox, alias, group, or contact exists and can receive external mail.
  3. Keep the full SMTP reply with the contact record so support can distinguish invalid addresses from policy blocks.

App handling

  • Classify as a hard bounce only when this provider-specific fingerprint matches.
  • Stop retries for the bounced recipient and keep the raw SMTP reply for audit and support.
  • Do not suppress other recipients in the same send unless they return their own matching bounce.

Avoid

  • Do not suppress from the numeric code alone. Confirm the provider, SMTP stage, and diagnostic text first.
  • Do not keep retrying once the provider-specific evidence says the mailbox is invalid.
  • Do not apply this suppression decision to the same code from another provider without matching that provider's text.
Common causes
  • The address is misspelled, deleted, disabled, or not mail-enabled.
  • The recipient domain accepts mail, but the specific mailbox cannot receive this message.
  • An alias, distribution group, or forwarded address no longer maps to a valid mailbox.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestampenvelope recipientcontact id
Classifier evidence
Specific cause
invalid recipient
Evidence
observed
support.apple.com source 1

Apple 550 5.1.1

Sender authorization

Fix sender setup

The receiving service refused the route, connector, or relay path. The recipient may still be valid, so fix routing before making a suppression decision.

Do not treat this as invalid recipient. Verify allowed sender/domain and SPF/DKIM/DMARC configuration.

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: high. Recipient validity: unknown.

Match this reply

SMTP stage
RCPT TO recipient check
Endpoint
Apple Private Relay
unauthorized sender

Start here

  1. 1

    Confirm that the sender is authorized for the destination service or relay.

  2. 2

    For Apple Private Relay, send only from domains registered and approved for relay use.

Confirm the cause

  1. Match provider, SMTP stage, endpoint, reply code, enhanced code, and diagnostic text.
  2. Run the sender, recipient, DNS, and content checks that match the diagnostic wording.
  3. Test with one recipient before retrying bulk traffic.

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
  • The provider-specific diagnostic text matches this class of failure.
  • The same SMTP code may have another meaning when the provider, stage, or endpoint differs.
  • Sender setup, recipient status, and provider policy should all be checked before taking irreversible action.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
Specific cause
apple private relay unauthorized sender
Evidence
official+observed
support.apple.com source 1

Apple 550 5.1.1

SMTP authentication or account configuration

Fix sender setup

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

On smtp.mail.me.com do not classify 5.1.1 as invalid recipient. Investigate iCloud identity, auth, and app-specific password.

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: low. Recipient validity: unknown.

Match this reply

SMTP stage
Authentication
Endpoint
SMTP submission

Start here

  1. 1

    Check SMTP credentials and account-level restrictions for the sending mailbox.

  2. 2

    Retry only after the account configuration has been corrected.

Confirm the cause

  1. Match provider, SMTP stage, endpoint, reply code, enhanced code, and diagnostic text.
  2. Run the sender, recipient, DNS, and content checks that match the diagnostic wording.
  3. Test with one recipient before retrying bulk traffic.

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
  • The provider-specific diagnostic text matches this class of failure.
  • The same SMTP code may have another meaning when the provider, stage, or endpoint differs.
  • Sender setup, recipient status, and provider policy should all be checked before taking irreversible action.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
Specific cause
icloud submission contextual 5 1 1
Evidence
observed
support.apple.com source 1

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