211
System status or help reply.
Common cause
The server is returning diagnostic or status information you asked for.
Good to know
Not an error — read the text that comes with it.
Free tool
SMTP error code lookup
A bounce message gives you a cryptic code and little else. Search the code below to see what it means, what usually causes it, and what to try next.
How to read a status code
Class — how serious it is Subject — what area failed Detail — the exact reason
The first digit tells you what to do next
- 2 — Success. The message was delivered.
- 4 — Temporary. Safe to retry; the server usually will.
- 5 — Permanent. Fix the cause before you resend.
A real bounce often shows two codes together, like 550 5.1.1. The 3-digit code (550) is the classic reply; the dotted code (5.1.1) is the precise enhanced status. You can paste either — or the whole line — into the search.
Try:
2xx
Success & status (2xx / 3xx)
Not errors — greetings, confirmations, and mid-conversation steps. If you see these, things are working.
214
Help reply.
Common cause
A response to the HELP command.
Good to know
Informational — usually points to the server’s documentation or a FAQ.
220
Service ready.
Common cause
The greeting the server sends when you connect.
Good to know
Nothing to do — it means the server is up and ready for commands.
221
Service closing transmission channel.
Common cause
The session ended cleanly after your mail was handed off.
Good to know
Normal end-of-session message. Nothing to do.
235
Authentication successful.
Common cause
Your AUTH login was accepted.
Good to know
You are logged in and can send.
250
Requested action completed.
Common cause
The command succeeded.
Good to know
On the final step this means the message was accepted for delivery — the opposite of an error.
251
User not local; will forward.
Common cause
The recipient is not hosted here, so the server will relay it onward.
Good to know
A normal relay action — nothing to fix.
252
Cannot verify recipient, will attempt delivery.
Common cause
The server cannot confirm the address exists but will try anyway.
Good to know
Usually fine — watch for a later bounce if the address turns out to be bad.
4xx
Temporary failures (4xx)
The server could not act right now but the message is not lost. It is safe to retry, and most servers will retry for you.
420
TCP read/write timeout — non-standard, mostly GroupWise.
Common cause
The connection to the remote server stalled mid-transfer, usually a Path MTU or firewall problem. The standard transient code is 421.
Quick fix
Retry, then fix Path MTU Discovery or firewall/ICMP filtering on the sending host.
447
Delivery time expired — a misread of 4.4.7, not a real code.
Enhanced code: 4.4.7 Common cause
Your server retried delivery for hours but each attempt hit a temporary failure until the message expired in the queue.
Quick fix
Fix the underlying transient failure: recipient MX/DNS, connectivity, or a backed-up queue.
449
Exchange routing error — non-standard, legacy Exchange only.
Common cause
Exchange could not route the message, usually a deleted routing group or a misconfigured SMTP connector.
Quick fix
Correct the connector and routing-group configuration.
458
Unable to queue messages for node — the ETRN failure reply.
Common cause
The relay you asked to flush queued mail could not queue for that host, often because it does not relay for that domain.
Quick fix
Confirm the relay serves that node; retry ETRN later.
471
Local filter or Exchange block — not a valid SMTP code.
Common cause
Your own server stopped the message: an outbound anti-spam/antivirus filter, or an Exchange database issue.
Quick fix
Read the full log, then tune the named filter or repair the Exchange store.
422
Mailbox full — a misread of enhanced status 4.2.2, not a real code.
Enhanced code: 4.2.2 Common cause
The recipient mailbox is over quota. The real bounce is 452 4.2.2.
Quick fix
Treat as a temporary soft bounce: retry with backoff until the recipient frees space.
441
No answer from host — a misread of 4.4.1, or an AuthSMTP quota code.
Enhanced code: 4.4.1 Common cause
Your server could not reach the recipient mail host, or your AuthSMTP account is over quota.
Quick fix
Retry; if it persists, check the recipient MX reachability and your outbound port 25.
442
Connection dropped during transmission — a tool label, not a real code.
Enhanced code: 4.4.2 Common cause
The session died mid-send. The standard form is a 421 with enhanced status 4.4.2.
Quick fix
Retry, raise SMTP timeouts, and stabilize the network path.
446
Maximum hop count exceeded — a label for a mail loop, not a real code.
Enhanced code: 4.4.6 Common cause
The message passed through too many servers, almost always a forwarding loop. The real bounce is 554 5.4.6.
Quick fix
Find and break the forwarding or connector loop.
421
Service not available, closing channel.
Enhanced code: 4.7.0 Common cause
Too many simultaneous connections, a rate limit, or a temporary server problem.
Quick fix
Back off and retry later; if it is constant, check you are not exceeding connection or sending limits.
450
Mailbox unavailable (temporary).
Enhanced code: 4.2.1 Common cause
The mailbox is busy or locked, or you are being greylisted / throttled on reputation.
Quick fix
Retry later — greylisting usually clears on the second attempt.
451
Requested action aborted — local error in processing.
Enhanced code: 4.3.0 Common cause
A processing error on the receiving side, often a content or anti-spam check, or overload.
Quick fix
Retry; if it persists it is usually a reputation or filter issue worth investigating.
452
Insufficient system storage or too many recipients.
Enhanced code: 4.5.3 Common cause
The server is out of resources, or you sent too many recipients at once.
Quick fix
Retry, and split large recipient lists into smaller batches.
431
Server out of memory or disk (temporary).
Enhanced code: 4.3.1 Common cause
The receiving server is temporarily out of resources.
Quick fix
Retry later with backoff.
432
Recipient incoming mail queue stopped (Exchange).
Enhanced code: 4.3.2 Common cause
The recipient mail queue is paused on the receiving side.
Quick fix
Retry later; the recipient admin resumes the queue.
454
Temporary authentication failure, or TLS not available.
Enhanced code: 4.7.0 Common cause
The server could not complete AUTH or STARTTLS right now (RFC 4954).
Quick fix
Retry; verify TLS is available and credentials are correct.
455
Server unable to accommodate parameters.
Common cause
A valid command carried parameters the server cannot handle right now (RFC 5321).
Quick fix
Retry, or drop the unsupported ESMTP parameter.
5xx
Permanent failures (5xx)
The server refused the message and will not try again. Fix the underlying cause before you resend.
511
Bad destination mailbox address — a misread of 5.1.1, not a real code.
Enhanced code: 5.1.1 Common cause
The recipient mailbox does not exist. The real bounce is 550 5.1.1.
Quick fix
Correct the recipient address and remove dead addresses from your list.
513
Non-standard label: bad address syntax, or a relay/auth failure.
Enhanced code: 5.1.3 Common cause
Vendors disagree: some map 513 to bad address syntax (5.1.3), others to authentication or relay denial.
Quick fix
Read the full bounce text, then fix the malformed address or the authentication.
517
Non-standard label: bad sender attribute, or a policy block.
Common cause
Not an RFC code. Vendors disagree: some map 517 to a malformed sender mail attribute (often Exchange), others to the recipient blocking the sender on policy.
Quick fix
Read the full server text, then fix the sender address attribute or check your reputation.
522
Mailbox full — a non-standard label for 552 / 5.2.2.
Enhanced code: 5.2.2 Common cause
The recipient mailbox is over its storage quota. The real code is 552 5.2.2.
Quick fix
The recipient must free space or raise their quota; stop retrying and verify the address.
533
Non-standard label, usually a misread of 5.3.3.
Enhanced code: 5.3.3 Common cause
Not an RFC code. Usually enhanced status 5.3.3, the destination cannot support a requested feature.
Quick fix
Read the full server text; if it is 5.3.3, stop requesting the unsupported SMTP extension.
510
Invalid recipient address — a vendor label, not a real code.
Enhanced code: 5.1.1 Common cause
A vendor synonym for a nonexistent or malformed recipient; the real bounce is 550 5.1.1 or 553.
Quick fix
Correct or remove the recipient address; read the real code in the bounce.
512
Domain not found — a vendor label for enhanced status 5.1.2.
Enhanced code: 5.1.2 Common cause
The recipient domain after the @ cannot be resolved or has no mail server.
Quick fix
Fix the domain typo or the recipient domain MX / DNS records.
515
Invalid recipient address — not a real RFC code.
Enhanced code: 5.1.1 Common cause
An error-list label for an invalid or nonexistent recipient; the real codes are 550, 551, or 553.
Quick fix
Correct the recipient address; act on the real code in the bounce.
521
Host does not accept mail.
Enhanced code: 5.3.2 Common cause
The server answers at connection time to signal it never accepts inbound mail (RFC 7504).
Quick fix
Send only to a domain with valid MX records and a listening SMTP server.
523
Message length exceeds administrative limit — enhanced 5.2.3 flattened.
Enhanced code: 5.2.3 Common cause
The message is larger than the recipient per-mailbox size limit; the real reply code is 552.
Quick fix
Reduce total message size or link large attachments.
531
Not a real code — usually mail-system-full (4.3.1 / 552) or an auth failure.
Common cause
A relabeling of enhanced status 4.3.1, or of a 530 / 535 authentication failure.
Quick fix
Read the raw code: free space or retry for storage, or fix credentials for auth.
534
Authentication mechanism too weak.
Enhanced code: 5.7.9 Common cause
The SASL mechanism you offered is weaker than server policy allows (RFC 4954).
Quick fix
Use OAuth2 or an App Password over TLS.
538
Encryption required for the requested authentication mechanism.
Enhanced code: 5.7.11 Common cause
You tried to authenticate over an unencrypted connection (RFC 4954).
Quick fix
Enable STARTTLS on 587 or implicit TLS on 465 before AUTH.
540
Not a real code — usually 5.4.0, or a provider literal rejection.
Enhanced code: 5.4.0 Common cause
Either enhanced status 5.4.0 (undefined network / routing) or a provider string such as Rambler Inactive / Blocked.
Quick fix
Read the bounce text; verify the recipient and your sender reputation.
542
Not a real SMTP code — a blog "bad connection" label.
Common cause
No RFC defines 542; the real rejection is a different 4xx / 5xx in your logs.
Quick fix
Find the actual code and enhanced status and act on that.
543
Not a real code — a mislabel of enhanced status 5.4.4 "unable to route".
Enhanced code: 5.4.4 Common cause
A routing or DNS failure rendered as a fake 543.
Quick fix
Verify the recipient domain MX / DNS records resolve.
546
Not a real code — a label for a mail loop (554 5.4.6).
Enhanced code: 5.4.6 Common cause
A message was forwarded in a loop until it passed the hop limit.
Quick fix
Remove the circular forwarding rule, alias, or auto-responder.
547
Delivery time expired — a garbled form of 5.4.7, not a real code.
Enhanced code: 5.4.7 Common cause
Your server retried for days, never reached the recipient, and bounced the message.
Quick fix
Check recipient MX / DNS and reachability, and your own queue.
555
MAIL FROM / RCPT TO parameters not recognized or not implemented.
Enhanced code: 5.5.4 Common cause
An unsupported ESMTP parameter or malformed envelope command (RFC 5321).
Quick fix
Fix the envelope syntax and drop unsupported extension parameters.
556
Domain does not accept mail (null MX).
Enhanced code: 5.1.10 Common cause
The recipient domain publishes a null MX record and accepts no email (RFC 7504 / 7505).
Quick fix
Verify the recipient domain; only its owner can publish valid MX records.
500
Syntax error — command unrecognized.
Common cause
A malformed command, sometimes a firewall or antivirus mangling the SMTP stream.
Quick fix
Check your mail client or relay config and any security software sitting between you and the server.
501
Syntax error in parameters or arguments.
Enhanced code: 5.5.4 Common cause
Usually a malformed email address or a bad command argument.
Quick fix
Verify the sender and recipient addresses are correctly formatted.
502
Command not implemented.
Common cause
The server does not support the command your client issued.
Quick fix
Check your client config or the server’s advertised capabilities.
503
Bad sequence of commands.
Common cause
Commands were sent out of order, or authentication is required before this step.
Quick fix
Make sure your client authenticates and follows the correct SMTP handshake order.
504
Command parameter not implemented.
Common cause
A parameter you passed is not supported by the server.
Quick fix
Check the command syntax your client is using.
530
Authentication required.
Enhanced code: 5.7.0 Common cause
You tried to send without logging in, or STARTTLS is required first.
Quick fix
Enable SMTP authentication (username/password or API key) in your client.
535
Authentication failed.
Enhanced code: 5.7.8 Common cause
Wrong credentials, or an app password / API key that was not accepted.
Quick fix
Re-check credentials — many providers require an app-specific password or API key rather than your normal login.
541
Recipient address rejected the message.
Enhanced code: 5.7.1 Common cause
The recipient’s spam filter refused it (non-standard code).
Quick fix
Content or reputation triggered a block — improve sender reputation and ask the recipient to whitelist you.
550
Mailbox unavailable / address rejected.
Enhanced code: 5.1.1 Common cause
Most often the recipient address does not exist; can also be a policy or reputation block.
Quick fix
Verify the address is correct and still active. If it does exist, the rejection is reputation-based.
551
User not local; relay denied.
Enhanced code: 5.7.1 Common cause
The server will not relay mail for your address.
Quick fix
Send through a server you are authorized to relay on — authenticate to your own provider.
552
Storage allocation exceeded / message too large.
Enhanced code: 5.2.2 Common cause
The recipient’s mailbox is full, or the message exceeds the size limit.
Quick fix
Reduce attachment size; if the mailbox is full, reach the recipient another way.
553
Mailbox name not allowed / invalid address.
Enhanced code: 5.1.3 Common cause
The recipient address is malformed or not permitted.
Quick fix
Check the address for typos and formatting errors.
554
Transaction failed.
Enhanced code: 5.7.1 Common cause
A catch-all permanent rejection — usually the message was flagged as spam or the sending IP is blacklisted.
Quick fix
Check blacklists for your IP and domain, then repair and warm up sender reputation.
X.0.x
Other / undefined
The server reported only a general failure class with no specifics. Read the full bounce text for the real reason.
X.1.x
Addressing
Something is wrong with the sender or recipient address itself.
X.1.0
Other address status
Common cause
Unspecified problem with sender or recipient address
Quick fix
Verify the complete email address and domain
X.1.1
Bad destination mailbox address
Common cause
Recipient mailbox does not exist
Quick fix
Check for typos and verify the recipient address
X.1.2
Bad destination system address
Common cause
Recipient domain does not accept mail or is invalid
Quick fix
Check the domain and its MX/DNS records
X.1.3
Bad destination mailbox address syntax
Common cause
Recipient address is malformed
Quick fix
Correct the address format
X.1.4
Destination mailbox address ambiguous
Common cause
Address maps to more than one recipient
Quick fix
Use the recipient's exact mailbox address
X.1.5
Destination address valid
Common cause
Positive delivery status
Good to know
No fix needed
X.1.6
Destination mailbox moved, no forwarding address
Common cause
Mailbox existed previously but no longer accepts mail
Quick fix
Get the recipient's current address
X.1.7
Bad sender mailbox address syntax
Common cause
Sender address is malformed
Quick fix
Correct the From / envelope sender address
X.1.8
Bad sender system address
Common cause
Sender domain is invalid or cannot accept return mail
Quick fix
Check sender-domain DNS and address configuration
X.1.9
Relayed to non-compliant mailer
Common cause
Message was forwarded to a system that cannot report more detail
Quick fix
Check the downstream relay or destination system
X.1.10
Recipient address has null MX
Common cause
Recipient domain explicitly does not accept email
Quick fix
Do not retry unless the recipient domain changes its mail configuration
X.2.x
Mailbox
The recipient exists, but their mailbox can't accept the message right now.
X.2.0
Other or undefined mailbox status
Common cause
Mailbox exists, but an unspecified mailbox problem occurred
Quick fix
Check the detailed bounce text or contact the recipient
X.2.1
Mailbox disabled
Common cause
Recipient mailbox exists but is not accepting mail
Quick fix
Wait if temporary or contact the recipient another way
X.2.2
Mailbox full
Common cause
Recipient exceeded mailbox quota
Quick fix
Retry later or ask the recipient to free space
X.2.3
Message length exceeds mailbox limit
Common cause
Message is too large for that mailbox
Quick fix
Reduce attachments / message size and resend
X.2.4
Mailing list expansion problem
Common cause
Distribution list could not expand its members
Quick fix
Retry later or contact the list administrator
X.3.x
Mail system
The problem is on the receiving mail system, not with your message or address.
X.3.0
Other or undefined mail system status
Common cause
Unspecified issue on destination mail system
Quick fix
Check the server response and retry if temporary
X.3.1
Mail system full
Common cause
Destination server storage is exhausted
Quick fix
Retry later
X.3.2
System not accepting network messages
Common cause
Destination server is overloaded, down, or under maintenance
Quick fix
Retry later or contact the receiving administrator
X.3.3
System not capable of selected features
Common cause
Destination cannot support a requested mail feature
Quick fix
Remove or change the unsupported feature
X.3.4
Message too big for system
Common cause
Message exceeds receiving system's maximum size
Quick fix
Reduce message or attachment size
X.3.5
System incorrectly configured
Common cause
Receiving mail system configuration prevents delivery
Quick fix
Recipient administrator needs to fix the server
X.3.6
Requested priority changed
Common cause
Message was accepted but priority was altered
Good to know
Usually no action required
X.4.x
Network & routing
The message couldn't find or reach the destination server.
X.4.0
Other or undefined network / routing status
Common cause
Unspecified network or routing problem
Quick fix
Check DNS, routing, and full server response
X.4.1
No answer from host
Common cause
Remote server did not respond
Quick fix
Retry later and check destination-server availability
X.4.2
Bad connection
Common cause
Connection timed out or degraded during delivery
Quick fix
Retry and check connectivity / TLS / network quality
X.4.3
Directory server failure
Common cause
DNS or another directory service was unavailable
Quick fix
Retry after DNS / directory service recovers
X.4.4
Unable to route
Common cause
No usable route to destination was found
Quick fix
Check recipient-domain DNS and MX records
X.4.5
Mail system congestion
Common cause
Destination system is overloaded
Quick fix
Retry later
X.4.6
Routing loop detected
Common cause
Mail keeps forwarding between servers
Quick fix
Fix forwarding, MX, or routing configuration
X.4.7
Delivery time expired
Common cause
Message remained undelivered until retry period expired
Quick fix
Investigate the earlier delivery failures before resending
X.5.x
Protocol
The SMTP conversation itself broke down — a command or protocol issue.
X.5.0
Other or undefined protocol status
Common cause
Unspecified SMTP / protocol problem
Quick fix
Check the full SMTP response and client / server compatibility
X.5.1
Invalid command
Common cause
SMTP command was unsupported or sent out of sequence
Quick fix
Fix SMTP command order or client configuration
X.5.2
Syntax error
Common cause
SMTP command syntax could not be parsed
Quick fix
Correct the malformed SMTP command
X.5.3
Too many recipients
Common cause
Message contains more recipients than server allows
Quick fix
Split recipients into smaller batches
X.5.4
Invalid command arguments
Common cause
Valid SMTP command contained unsupported / invalid values
Quick fix
Correct the command parameters
X.5.5
Wrong protocol version
Common cause
Client and server use incompatible protocol versions
Quick fix
Update or reconfigure the SMTP client / server
X.5.6
Authentication exchange line too long
Common cause
AUTH response exceeds server buffer / mechanism limit
Quick fix
Use a supported authentication mechanism or update the client
X.6.x
Content & media
The message content, format, or encoding caused the delivery problem.
X.6.0
Other or undefined media error
Common cause
Message content caused an unspecified delivery problem
Quick fix
Inspect content format and full bounce response
X.6.1
Media not supported
Common cause
Receiver cannot process the message content type
Quick fix
Resend using supported MIME / content formats
X.6.2
Conversion required and prohibited
Common cause
Delivery requires content conversion that policy forbids
Quick fix
Change message format or conversion policy
X.6.3
Conversion required but unsupported
Common cause
Gateway cannot perform required message conversion
Quick fix
Send using a more compatible format
X.6.4
Conversion with loss performed
Common cause
Message delivered after conversion lost some data
Good to know
Usually no fix needed unless the loss is unacceptable
X.6.5
Conversion failed
Common cause
Required message conversion was unsuccessful
Quick fix
Change content format or fix the conversion gateway
X.6.6
Message content not available
Common cause
Server could not retrieve externally stored content
Quick fix
Restore / access the content source and retry
X.6.7
Non-ASCII addresses not permitted
Common cause
Server / client does not support internationalized mailbox address
Quick fix
Use an ASCII address or SMTPUTF8-capable infrastructure
X.6.8
UTF-8 reply required but not permitted
Common cause
Server needs SMTPUTF8 response support the client lacks
Quick fix
Use an SMTPUTF8-capable client
X.6.9
UTF-8 header cannot be transferred
Common cause
One or more recipients cannot receive the SMTPUTF8 message
Quick fix
Use compatible recipient infrastructure or ASCII headers
X.6.10
Deprecated
Common cause
Duplicate of X.6.8
Quick fix
Treat as X.6.8
X.7.x
Security & policy
Authentication, encryption, or the receiver's policy blocked the message. This is where reputation problems show up.
X.7.0
Other or undefined security status
Common cause
Unspecified security or policy rejection
Quick fix
Read the full rejection text and check receiver policy
X.7.1
Delivery not authorized
Common cause
Sender is blocked by recipient / host policy
Quick fix
Check authentication, reputation, allowlists, and recipient policy
X.7.2
Mailing list expansion prohibited
Common cause
Sender is not permitted to post to the list
Quick fix
Request permission or use an authorized sender
X.7.3
Security conversion required but impossible
Common cause
Secure-message conversion cannot be performed
Quick fix
Align encryption / security formats between systems
X.7.4
Security features not supported
Common cause
Receiver does not support requested security feature
Quick fix
Use a supported authentication / security method
X.7.5
Cryptographic failure
Common cause
Required key or certificate is unavailable or invalid
Quick fix
Fix certificates, keys, or encryption configuration
X.7.6
Cryptographic algorithm not supported
Common cause
Receiver does not support required algorithm
Quick fix
Use a mutually supported cryptographic algorithm
X.7.7
Message integrity failure
Common cause
Message was corrupted or altered
Quick fix
Check signing, transport, and message modification
X.7.8
Authentication credentials invalid
Common cause
Username, password, or token is incorrect or insufficient
Quick fix
Update SMTP credentials and authenticate again
X.7.9
Authentication mechanism too weak
Common cause
Server requires a stronger AUTH method
Quick fix
Switch to a stronger supported authentication mechanism
X.7.10
Encryption needed
Common cause
AUTH method requires a secure transport layer
Quick fix
Enable TLS before authenticating
X.7.11
Encryption required for authentication mechanism
Common cause
Selected AUTH method cannot be used without encryption
Quick fix
Connect using TLS or choose another AUTH method
X.7.12
Password transition needed
Common cause
Account must migrate to a new authentication method
Quick fix
Complete the required password / authentication transition
X.7.13
User account disabled
Common cause
SMTP account has been disabled
Quick fix
Contact the provider or admin to restore the account
X.7.14
Trust relationship required
Common cause
Submission server requires configured trust with another service
Quick fix
Configure the required trust relationship
X.7.15
Priority level too low
Common cause
Server currently refuses mail below a priority threshold
Quick fix
Increase priority if appropriate or retry later
X.7.16
Message too big for specified priority
Common cause
Size exceeds the limit for that priority class
Quick fix
Reduce size, raise priority if appropriate, or retry later
X.7.17
Mailbox owner has changed
Common cause
Recipient mailbox changed ownership after the specified RRVS date
Quick fix
Verify the intended recipient before resending
X.7.18
Domain owner has changed
Common cause
Recipient domain ownership changed after the RRVS date
Quick fix
Verify the destination before resending
X.7.19
RRVS test cannot be completed
Common cause
Receiver lacks timestamp data required for RRVS validation
Quick fix
Decide whether to resend without RRVS protection
X.7.20
No passing DKIM signature found
Common cause
Message has no DKIM signature that validates successfully
Quick fix
Fix DKIM signing, selector, DNS record, and message-modification issues
X.7.21
No acceptable DKIM signature found
Common cause
DKIM passes technically but the receiver does not accept the signature
Quick fix
Check signing domain, selector, alignment, and receiver policy
X.7.22
No valid author-matched DKIM signature
Common cause
Passing DKIM signature does not match the From domain
Quick fix
Configure DKIM alignment with the author domain
X.7.23
SPF validation failed
Common cause
SPF returned fail
Quick fix
Correct the SPF record and authorized sending sources
X.7.24
SPF validation error
Common cause
SPF evaluation itself failed due to a DNS or configuration error
Quick fix
Check SPF syntax, DNS availability, and lookup limits
X.7.25
Reverse DNS validation failed
Common cause
Sending IP failed the receiver's PTR / rDNS check
Quick fix
Configure valid PTR and matching forward DNS
X.7.26
Multiple authentication checks failed
Common cause
More than one authentication mechanism failed
Quick fix
Check SPF, DKIM, DMARC, and related authentication together
X.7.27
Sender address has null MX
Common cause
Sender domain explicitly declares it receives no mail
Quick fix
Use a valid return-path domain that accepts mail
X.7.28
Mail flood detected
Common cause
Message resembles a large flood of abusive mail
Quick fix
Stop excessive sending, investigate compromise, and reduce / review traffic
X.7.29
ARC validation failure
Common cause
ARC chain did not validate
Quick fix
Check ARC seals / signatures and intermediary modifications
X.7.30
REQUIRETLS support required
Common cause
Next-hop server cannot meet the message's REQUIRETLS requirement
Quick fix
Use a route that supports REQUIRETLS or remove the requirement if appropriate
No codes match that
Try just the code (like 550 or 4.2.2) or a single
keyword. If you pasted a full bounce line, keep only the code part.
Ready when you are
Most rejections come back to sender reputation.
Auth failures, policy blocks, and flood flags (the X.7.x and 5xx codes) are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.