SMTP enhanced status codes and provider bounce codes
An enhanced status code such as 5.1.1 has three parts: the class (2 success, 4 temporary, 5 permanent), the subject (1 address, 2 mailbox, 3 mail system, 4 network, 5 protocol, 6 content, 7 security or policy) and the detail. This index has one page for every code in the IANA registry, and one for every code Gmail, Microsoft 365 and Yahoo document themselves.
Have a bounce in front of you? Paste it into the bounce message analyzer and it finds the codes for you. The common basic-plus-enhanced pairs, such as 550 5.1.1, have full guides on the SMTP codes page.
The IANA registry, by subject
81 codes, as listed in the IANA SMTP Enhanced Status Codes registry. Each page shows the class it is used in.
X.0.x Other or undefined
X.1.x Addressing
- 5.1.0 Other address status
- 5.1.1 Bad destination mailbox address
- 5.1.2 Bad destination system address
- 5.1.3 Bad destination mailbox address syntax
- 5.1.4 Destination mailbox address ambiguous
- 2.1.5 Destination address valid
- 5.1.6 Destination mailbox has moved, No forwarding address
- 5.1.7 Bad sender's mailbox address syntax
- 5.1.8 Bad sender's system address
- 5.1.9 Message relayed to non-compliant mailer
- 5.1.10 Recipient address has null MX
X.2.x Mailbox
X.3.x Mail system
X.4.x Network and routing
X.5.x Mail delivery protocol
X.6.x Message content or media
- 5.6.0 Other or undefined media error
- 5.6.1 Media not supported
- 5.6.2 Conversion required and prohibited
- 5.6.3 Conversion required but not supported
- 2.6.4 Conversion with loss performed
- 5.6.5 Conversion Failed
- 5.6.6 Message content not available
- 5.6.7 Non-ASCII addresses not permitted for that sender/recipient
- 5.6.8 UTF-8 string reply is required, but not permitted by the SMTP client
- 5.6.9 UTF-8 header message cannot be transferred to one or more recipients, so the message must be rejected
- 5.6.10 Deprecated duplicate of X.6.8
X.7.x Security or policy
- 5.7.0 Other or undefined security status
- 5.7.1 Delivery not authorized, message refused
- 5.7.2 Mailing list expansion prohibited
- 5.7.3 Security conversion required but not possible
- 5.7.4 Security features not supported
- 5.7.5 Cryptographic failure
- 5.7.6 Cryptographic algorithm not supported
- 5.7.7 Message integrity failure
- 5.7.8 Authentication credentials invalid
- 5.7.9 Authentication mechanism is too weak
- 5.7.10 Encryption Needed
- 5.7.11 Encryption required for requested authentication mechanism
- 4.7.12 A password transition is needed
- 5.7.13 User Account Disabled
- 5.7.14 Trust relationship required
- 4.7.15 Priority Level is too low
- 5.7.16 Message is too big for the specified priority
- 5.7.17 Mailbox owner has changed
- 5.7.18 Domain owner has changed
- 5.7.19 RRVS test cannot be completed
- 5.7.20 No passing DKIM signature found
- 5.7.21 No acceptable DKIM signature found
- 5.7.22 No valid author-matched DKIM signature found
- 5.7.23 SPF validation failed
- 5.7.24 SPF validation error
- 5.7.25 Reverse DNS validation failed
- 5.7.26 Multiple authentication checks failed
- 5.7.27 Sender address has null MX
- 5.7.28 Mail flood detected
- 5.7.29 ARC validation failure
- 5.7.30 REQUIRETLS support required
Gmail bounce codes
57 codes returned by Gmail and Google Workspace, as documented in Google Workspace Admin Help: Gmail SMTP errors and codes, with their exact wording.
- 450 4.2.1 Recipient receiving mail too fast
- 452 4.2.2 Recipient inbox out of storage
- 451 4.3.0 Temporarily rejected
- 421 4.3.0 Temporary system problem
- 451 4.4.2 Timeout, closing connection
- 421 4.4.5 Server busy
- 451 4.5.0 SMTP protocol violation
- 452 4.5.3 Too many recipients
- 421 4.7.0 Rate limited or connection closed
- 454 4.7.0 Too many sign-in attempts (SMTP relay)
- 451 4.7.23 Rate limited: missing PTR record
- 451 4.7.24 Rate limited: suspicious SPF record
- 451 4.7.26 DMARC check hit a temporary DNS failure
- 421 4.7.26 Rate limited: unauthenticated
- 421 4.7.27 Rate limited: SPF did not pass
- 421 4.7.28 Rate limited: unusual rate of mail
- 421 4.7.29 Rate limited: no TLS
- 421 4.7.30 Rate limited: DKIM did not pass
- 421 4.7.40 Rate limited: no DMARC policy
- 421 4.7.32 Rate limited: From not aligned
- 421 5.7.32 Blocked: From not aligned
- 550 5.1.1 Account does not exist
- 553 5.1.2 Recipient domain not found
- 553 5.1.3 Recipient address not valid
- 553 5.1.7 Sender address not valid
- 550 5.2.1 Account inactive, or receiving too fast
- 552 5.2.2 Inbox out of storage and inactive
- 552 5.3.4 Message, attachment or header too large
- 550 5.4.5 Daily sending limit exceeded
- 554 5.4.6 Message exceeded 50 hops
- 503 5.5.1 Bad sequence of commands
- 502 5.5.1 Unrecognized or unimplemented command
- 501 5.5.2 Cannot decode response
- 555 5.5.2 Syntax error
- 550 5.5.3 Too many recipients for this sender
- 501 5.5.4 Invalid HELO/EHLO argument
- 554 5.6.0 Message malformed
- 552 5.7.0 Blocked: content is a security risk
- 503 5.7.0 No identity changes permitted
- 550 5.7.0 SMTP relay denied or sending denied
- 554 5.7.0 Too many unauthenticated commands
- 530 5.7.0 Authentication required or STARTTLS first
- 550 5.7.1 Blocked by policy, reputation or headers
- 523 5.7.10 No pipelining after STARTTLS
- 501 5.7.11 Syntax error, no parameters allowed
- 534 5.7.14 Sign in through a web browser
- 550 5.7.24 Suspicious SPF record
- 550 5.7.25 Blocked: missing PTR record
- 550 5.7.26 Blocked: unauthenticated or DMARC failure
- 550 5.7.27 Blocked: SPF did not pass
- 550 5.7.28 Blocked: unusual rate of unsolicited mail
- 550 5.7.29 Blocked: no TLS
- 550 5.7.30 Blocked: DKIM did not pass
- 550 5.7.40 Blocked: no DMARC policy
- 504 5.7.40 Unrecognized authentication type
- 535 5.7.80 Username and password not accepted
- 534 5.7.90 App password required
Microsoft 365 bounce codes
89 codes returned by Microsoft 365, Exchange Online and Outlook.com, as documented in Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, with their exact wording.
- 432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded
- 4.4.7 Message expired
- 4.4.8 MX hosts of <domain> failed MTA-STS validation
- 4.4.316 Connection refused [Message=Socket error code 10061]
- 450 4.4.317 Cannot connect to remote server [Message=UntrustedRoot]
- 4.5.3 Too many recipients
- 4.7.5 Remote certificate failed MTA-STS validation
- 4.7.26 Access denied, a message sent over IPv6 [2a01:111:f200:2004::240] must pass either SPF or DKIM validation, this message is not signed
- 4.7.321 starttls-not-supported: Destination mail server must support TLS to receive mail
- 4.7.322 certificate-expired: Destination mail server's certificate is expired
- 4.7.323 tlsa-invalid: The domain failed DANE validation
- 4.7.324 dnssec-invalid: Destination domain returned invalid DNSSEC records
- 4.7.325 certificate-host-mismatch: Remote certificate MUST have a common name or subject alternative name that matches the hostname (DANE)
- 4.7.500-699 Access denied, please try again later
- 4.7.850-899 Access denied, please try again later
- 5.0.350 Generic error, x-dg-ref header is too long, or Requested action not taken: policy violation detected (AS345)
- 5.1.0 Sender denied
- 5.1.1 Bad destination mailbox address
- 5.1.8 Access denied, bad outbound sender
- 5.1.10 Recipient not found
- 550 5.1.20 Multiple From addresses are not allowed without Sender address
- 5.1.90 Your message can't be sent because you've reached your daily limit for message recipients
- 5.2.2 Submission quota exceeded
- 5.2.121 Recipient's per hour message receive limit from specific sender exceeded
- 5.2.122 Recipient's per hour message receive limit exceeded
- 5.3.190 Journaling on-premises messages to Microsoft 365 or Office 365 not supported when Journaling Archive is disabled
- 5.4.1 Relay Access Denied / Recipient address rejected: Access denied
- 5.4.6 / 5.4.14 Routing loop detected
- 5.4.8 MX hosts of <domain> failed MTA-STS validation
- 5.4.300 Message expired
- 5.5.0 550 5.5.0 Requested action not taken: mailbox unavailable
- 5.6.11 Invalid characters
- 5.7.1 Delivery not authorized / Unable to relay / Client was not authenticated
- 5.7.5 Remote certificate failed MTA-STS validation
- 5.7.12 Sender was not authenticated by organization
- 5.7.23 The message was rejected because of Sender Policy Framework violation
- 5.7.25 Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record
- 5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM
- 5.7.64 TenantAttribution; Relay Access Denied
- 5.7.124 Sender not in allowed-senders list
- 5.7.133 Sender not authenticated for group
- 5.7.134 Sender was not authenticated for mailbox
- 5.7.135 Sender was not authenticated for public folder
- 5.7.136 Sender was not authenticated
- 5.7.232 Your message can't be sent because your trial tenant has exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)
- 5.7.233 Your message can't be sent because your tenant exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)
- 5.7.236 Your message can't be sent because your tenant has exceeded its daily limit for sending email to external recipients from your tenant's onmicrosoft.com domains
- 5.7.321 starttls-not-supported: Destination mail server must support TLS to receive mail
- 5.7.322 certificate-expired: Destination mail server's certificate is expired
- 5.7.323 tlsa-invalid: The domain failed DANE validation
- 5.7.324 dnssec-invalid: Destination domain returned invalid DNSSEC records
- 5.7.325 certificate-host-mismatch: Remote certificate MUST have a common name or subject alternative name matching the hostname (DANE)
- 5.7.367 Remote server returned not permitted to relay
- 5.7.501 Access denied, spam abuse detected
- 5.7.502 Access denied, banned sender
- 5.7.503 Access denied, banned sender
- 5.7.504 [[email protected]]: Recipient address rejected: Access denied
- 5.7.505 Access denied, banned recipient
- 5.7.506 Access Denied, Bad HELO
- 5.7.507 Access denied, rejected by recipient
- 5.7.508 Access denied, [$SenderIPAddress] has exceeded permitted limits within $range range
- 5.7.509 Access denied, sending domain [$SenderDomain] does not pass DMARC verification and has a DMARC policy of reject
- 5.7.510 Access denied, [contoso.com] does not accept email over IPv6
- 5.7.511 Access denied, banned sender
- 5.7.512 Access denied, message must be RFC 5322 section 3.6.2 compliant
- 5.7.513 Service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607)
- 5.7.520 Access denied, Your organization does not allow external forwarding
- 5.7.606-649 Access denied, banned sending IP [IP1.IP2.IP3.IP4]
- 5.7.703 Your message can't be delivered because messages to XXX, YYY are blocked by your organization using Tenant Allow Block List
- 5.7.705 / 5.7.708 5.7.705 Access denied, tenant has exceeded threshold, 5.7.708 Access denied, traffic not accepted from this IP
- 5.7.750 Service unavailable
- 5.7.800 Access denied, banned sender
- 421 4.3.240 The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel
- 450 4.4.244 The HVE message can't be submitted because the recipient rate limit was exceeded
- 535 5.7.142 XOAUTH2 authentication failed
- 535 5.7.143 XOAUTH2 authentication failed
- 535 5.7.144 XOAUTH2 authentication failed
- 550 5.1.241 The message can't be delivered because the recipient is an HVE account
- 550 5.2.240 The email account is not set up properly for the HVE service
- 550 5.2.241 The HVE account is not properly set up for billing
- 550 5.6.240 One of MIME Headers is not allowed
- 550 5.6.241 Mandatory From Header is missing or invalid
- 550 5.7.240 The application is not allowed for use with an HVE account
- 550 5.7.241 Trial tenant is not able to use HVE
- 550 5.7.242 Inactive tenant is not able to use HVE
- 550 5.7.243 This tenant is blocked from using HVE
- 550 5.7.244 Message rejected
- 550 5.7.245 External sending is not supported for HVE accounts
- 550 5.7.246 Removed external recipients that were not originally specified on submission
Yahoo bounce codes
4 codes returned by Yahoo and AOL, as documented in Yahoo Sender Hub: SMTP Error Codes, with their exact wording.
Which of these does verification prevent?
The address codes, X.1.1 (no such mailbox), X.1.2 (no such domain), X.1.3 (bad syntax) and X.1.10 (null MX), are the ones a mailbox check finds before you send. Security and policy codes under X.7 are about your sending setup and are fixed in DNS and authentication, not in the list.
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- Yahoo Sender Hub: SMTP Error Codes (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked 2026-11-05