Every SMTP bounce code, searchable in one place.

220

Service Ready

The receiving mail server is ready to accept your connection. This is the standard SMTP greeting message sent when a server is available and willing to begin the mail transaction.

Success

221

Closing Connection

The mail server is closing the SMTP connection after completing the transaction. This is a normal response to the QUIT command and indicates the conversation has ended successfully.

Success

250

Requested Action Completed

The mail server has successfully accepted your request. This is the primary success response in SMTP, confirming that a command (HELO, MAIL FROM, RCPT TO, or DATA) was completed successfully.

Success

251

User Not Local - Will Forward

The recipient is not hosted on this server, but the server will forward the message to the correct destination. The email will still be delivered, but through an intermediary relay.

Success

252

Cannot Verify User

The server cannot verify whether the recipient address is valid but will attempt delivery anyway. This is a security measure to prevent address harvesting by spammers.

Success

354

Start Mail Input

The server is ready to receive the message body. This is the normal response after the DATA command, indicating you can begin transmitting the email content (headers and body).

Info

421

Service Not Available - Try Again Later

The receiving mail server is temporarily unable to process your email and has asked you to retry later. This soft bounce is commonly caused by rate limiting, server overload, greylisting, or temporary IP reputation issues with providers like Gmail and Microsoft.

Soft

422

Recipient Mailbox Storage Exceeded

The recipient mailbox has exceeded its storage limit and cannot accept new messages. This is a temporary failure that may resolve when the recipient frees up space by deleting old emails.

Soft

431

Not Enough Disk Space on Server

The receiving mail server has run out of disk space and cannot store incoming messages. This is a server-side issue that will typically be resolved by the server administrator.

Soft

441

No Response from Recipient Server

Your sending server could not get a response from the recipient mail server. The remote server may be down, unreachable, or the connection timed out before a response was received.

Soft

450

Mailbox Unavailable - Busy or Temporarily Blocked

The recipient mailbox is temporarily unavailable. This soft bounce can be caused by greylisting, temporary mailbox locks, server-side processing, or anti-spam measures that require senders to retry.

Soft

451

Local Error in Processing

The receiving server encountered a temporary internal error while processing your email. This is typically a server-side issue such as database failures, DNS lookup timeouts, or anti-spam processing errors that will resolve on retry.

Soft

452

Insufficient System Storage

The receiving mail server does not have enough storage to accept your message. This affects the entire server, not just one mailbox, and is typically caused by disk space issues that the server administrator must resolve.

Soft

500

Syntax Error - Command Unrecognized

The receiving server did not recognize the SMTP command sent by your mail server. This usually indicates a protocol compatibility issue, misconfigured sending server, or a firewall/security appliance interfering with the SMTP conversation.

Hard

501

Syntax Error in Parameters or Arguments

The SMTP command was recognized but contained invalid parameters. Common causes include malformed email addresses in MAIL FROM or RCPT TO commands, invalid characters, or improperly formatted command arguments.

Hard

502

Command Not Implemented

The receiving server does not support the SMTP command your server tried to use. This typically occurs when your server attempts to use an SMTP extension like STARTTLS or AUTH that the receiving server has not implemented.

Hard

503

Bad Sequence of Commands

SMTP commands were sent in the wrong order. The SMTP protocol requires commands in a specific sequence: HELO/EHLO, then MAIL FROM, then RCPT TO, then DATA. Sending commands out of order triggers this error.

Hard

504

Command Parameter Not Implemented

The SMTP command is supported but the specific parameter or extension used is not implemented by the receiving server. This commonly occurs with unsupported authentication mechanisms or encoding parameters.

Hard

521

Server Does Not Accept Mail

The receiving server does not accept any incoming email. The domain exists but its mail server is explicitly configured to reject all mail, or the domain has published a null MX record indicating it does not receive email.

Hard

530

Authentication Required

The server requires SMTP authentication before accepting email. Your sending server must provide valid credentials using the AUTH command before sending the MAIL FROM command.

Hard

535

Authentication Failed

SMTP AUTH was attempted and rejected. The username or password is wrong, expired, or not permitted for this connection. It is the single most common error when an application first tries to send.

Hard

541

Message Rejected for Policy Reasons

The receiving server rejected your email based on a content or security policy. This can be triggered by spam content, blacklisted URLs in the message body, attachment restrictions, or organizational security policies.

Hard

550

Requested Action Not Taken - Mailbox Unavailable

The recipient email address is invalid, does not exist, or has been blocked. This is the most common permanent bounce code and a strong signal that the address should be removed from your list immediately. Continued sending to 550 addresses damages sender reputation.

Hard

551

User Not Local - Please Try Another Path

The recipient is not hosted on this server and the server will not forward the message. Unlike 251 (which forwards), 551 tells the sender to retry with a different server or address, often providing the correct forwarding address.

Hard

552

Message Size Exceeds Fixed Limit

Your email message (including attachments) exceeds the maximum size limit set by the receiving server. Most providers allow 25-50MB but some organizations set lower limits. Reduce attachments or use file sharing links instead.

Hard

553

Mailbox Name Not Allowed

The recipient email address has a syntax problem, uses characters not allowed by the server, or the server policy does not allow that particular mailbox name. This can also indicate a relay attempt that the server refuses to process.

Hard

554

Transaction Failed

The email transaction has failed permanently. This catch-all permanent error is used when the server rejects the message for reasons including spam detection, content policy violations, IP blacklisting, authentication failures, or general policy blocks.

Hard

556

Domain Does Not Accept Mail

The recipient domain has explicitly declared it does not accept any email by publishing a null MX record. This is a permanent failure - the domain cannot receive email and the address should be removed from your list immediately.

Hard

5.0.0

Other Undefined Status

A catch-all permanent failure code used when the server cannot categorize the specific error. Check the accompanying SMTP reply code and message text for more details about the actual failure reason.

Hard

5.0.350

Generic Remote Server Error

Microsoft's catch-all for an error the recipient organisation returned that Exchange Online could not classify. The useful information is always in the text that follows, never in the code.

Hard

5.1.0

Other Address Status

A general address-related permanent failure. The recipient address has a problem that does not fit a more specific category. This often indicates the address is invalid or the domain cannot be reached for delivery.

Hard

5.1.1

Bad Destination Mailbox Address - User Unknown

The recipient email address does not exist on the destination server. The specific user mailbox is invalid, deleted, or was never created. This is the single most common bounce code and the address must be removed from your list immediately.

Hard

5.1.2

Bad Destination System Address - Domain Not Found

The recipient domain does not exist, has no MX records, or cannot be resolved through DNS. The entire domain (the part after @) is invalid, not just the user. Remove this address immediately.

Hard

5.1.3

Bad Destination Mailbox Address Syntax

The recipient email address has a syntax error that makes it unparseable. The address format violates RFC 5321 rules - it may contain illegal characters, missing @ symbol, spaces, or other formatting problems.

Hard

5.1.4

Destination Mailbox Address Ambiguous

The recipient address matches multiple mailboxes and the server cannot determine which one to deliver to. This is rare and typically occurs with misconfigured mail routing or directory services.

Hard

5.1.6

Destination Mailbox Has Moved

The recipient mailbox has been permanently moved to a new address and the server does not automatically forward mail. Update your records with the new address provided in the bounce response.

Hard

5.1.7

Bad Sender Mailbox Address Syntax

Your sender (From) email address has a syntax error. The receiving server rejected the message because the MAIL FROM address is malformed, contains invalid characters, or does not conform to email address standards.

Hard

5.1.8

Bad Sender System Address

Your sender domain does not exist or cannot be resolved. The receiving server looked up the domain in your From address and could not find it in DNS, meaning bounce messages cannot be returned to you.

Hard

5.1.10

Recipient Address Has Null MX

The recipient domain has explicitly declared that it accepts no email. A null MX is a deliberate statement, not a misconfiguration, so no address at that domain will ever receive mail.

Hard

5.1.90

Daily Recipient Limit Reached

The sending mailbox has reached its daily recipient limit in Microsoft 365. Numbered 5.x but functionally temporary, since the allowance resets, so the recipients must not be suppressed.

Soft

5.2.1

Mailbox Disabled or Not Accepting Messages

The recipient mailbox exists but has been disabled, suspended, or is not currently accepting messages. This can be a permanent or temporary condition depending on whether the account is suspended or permanently deactivated.

Hard

5.2.2

Mailbox Full - Over Quota

The recipient mailbox has exceeded its storage quota and cannot accept new messages. This is one of the most common soft bounce codes. The email may be delivered if the recipient deletes messages to free up space.

Soft

5.2.3

Message Length Exceeds Administrative Limit

Your email exceeds the maximum message size allowed by the recipient server. This includes all content: headers, body text, HTML, and attachments after Base64 encoding. Reduce the message size or use file sharing links.

Hard

5.2.121

Recipient Hourly Receive Limit From Sender Exceeded

You have sent one recipient more mail in an hour than Microsoft allows from a single sender. The limit is per sender-recipient pair, so it does not affect anyone else you write to.

Soft

5.2.122

Recipient Hourly Receive Limit Exceeded

The recipient is receiving more mail per hour than Microsoft allows, counting every source. The limit belongs to them, not to you, so slowing your own sending has limited effect.

Soft

4.2.2

Mailbox Full - Temporary

The recipient mailbox is temporarily over quota. The server is indicating this is a temporary condition and suggests retrying later. This is the soft bounce variant of the mailbox full error.

Soft

4.2.1

Mailbox Disabled or Rate Limited - Temporary

The recipient mailbox is not accepting messages right now. Gmail uses this specifically when a recipient is receiving mail faster than it will allow, which is a limit on them rather than on you.

Soft

4.3.1

Mail System Full

The receiving mail system has run out of storage space. This affects all mailboxes on the server, not just one recipient. It is a server-side issue that the administrator must resolve.

Soft

4.3.0

Other Mail System Status - Temporary

A temporary problem inside the receiving mail system that it has not described further. Gmail uses it for general temporary rejections and for a transaction addressing multiple destination domains.

Soft

4.3.2

System Not Accepting Messages

The mail system is temporarily not accepting new messages. This could be due to maintenance, system overload, or an administrative decision to pause incoming mail. Retry later.

Soft

4.4.1

No Answer from Host

The receiving mail server is not responding to connection attempts. The server may be down, unreachable due to network issues, or a firewall may be blocking the connection. Your MTA will retry automatically.

Soft

4.4.0

Other Network or Routing Status

A temporary network or routing problem the server has not described further. Something went wrong reaching the destination, and the sending system will retry.

Soft

4.4.2

Bad Connection

The connection was established but the transaction could not be completed. Something dropped the session partway through, most often a timeout or an idle connection being reaped.

Soft

4.4.5

Mail System Congestion

The receiving system is too busy to take the message right now. Gmail sends it as 421 4.4.5 server busy. It is about their capacity, not your reputation.

Soft

4.4.7

Delivery Time Expired

Your email has been sitting in the retry queue for too long and the maximum delivery time has expired. The sending server tried repeatedly to deliver the message but failed every attempt over the configured retry period (typically 24-72 hours).

Soft

4.5.3

Too Many Recipients - Temporary

The transaction named more recipients than the server will accept at once, or exceeded a size limit for a single transaction. Splitting the send into smaller batches resolves it.

Soft

4.7.0

Other Security or Policy Status - Temporary

The receiving server has temporarily rejected your email for security or policy reasons. This is heavily used by Gmail and Microsoft for IP reputation-based throttling. Repeated 4.7.0 responses indicate a sender reputation problem that needs attention.

Soft

4.7.1

Delivery Not Authorized - Temporary

The receiving server is refusing your message on security or policy grounds but is inviting a retry. Greylisting, a missing reverse DNS record, rate limiting, and temporary blocklist hits all surface as 4.7.1. It is a soft bounce and must never be suppressed like its permanent twin 5.7.1.

Soft

4.7.5

Cryptographic Failure - Temporary

A TLS or certificate problem stopped delivery, and the receiver is deferring rather than rejecting. Microsoft returns it when a remote certificate fails MTA-STS validation.

Soft

4.7.26

Multiple Authentication Checks Failed - Temporary

The message failed more than one authentication check and the receiver is deferring rather than rejecting. Both Google and Microsoft use this code to hold unauthenticated mail, and Microsoft applies it specifically to IPv6 senders.

Soft

4.7.28

Mail Flood Detected - Temporary Rate Limit

The receiver has decided your traffic looks like a flood and is throttling it. This is the code Gmail returns most often when it rate limits a sender, and it is a reputation signal rather than a capacity one.

Soft

4.7.27

Rate Limited - SPF Authentication Failed

Google is throttling your mail specifically because SPF did not pass. The message is deferred rather than refused, but the throttle stays until the SPF failure is fixed.

Soft

4.7.29

Rate Limited - Connection Not Encrypted

Google is throttling your mail because the connection did not use TLS. Opportunistic encryption is no longer optional at volume, and cleartext delivery now costs you throughput.

Soft

4.7.30

Rate Limited - DKIM Authentication Failed

Google is throttling your mail specifically because DKIM did not pass. Unlike a general reputation deferral, this one names a single mechanism, so the fix is unambiguous.

Soft

4.7.32

Rate Limited - From Header Not Aligned

The temporary form of an alignment failure. Google is throttling rather than blocking, which makes this the warning that arrives before 5.7.32 starts rejecting.

Soft

4.7.40

Rate Limited - DMARC Record Missing or Invalid

Google is throttling your mail because the sending domain publishes no readable DMARC record. It is the grace period before 5.7.40 begins rejecting outright.

Soft

5.7.1

Delivery Not Authorized - Message Refused

The receiving server has permanently rejected your email based on a security or policy decision. This is the most common policy-based rejection code and can be triggered by spam detection, IP blacklisting, content filtering, authentication failures, or explicit sender blocks.

Hard

5.7.0

Other Security or Policy Status

A permanent rejection on security or policy grounds where the receiver has not narrowed the reason further. Gmail uses it for at least ten distinct refusals, so the text after the code is the whole diagnosis.

Hard

5.7.4

Security Features Not Supported

The message asked for a security feature the receiving server cannot provide, so it was refused rather than delivered without it. Usually an unsupported SMTP extension or authentication mechanism.

Hard

5.7.5

Cryptographic Failure

A receiving system that was entitled to validate or decrypt your message could not do so. In practice this is almost always a TLS problem: an expired or mismatched certificate, a failed MTA-STS check, or a cipher the two servers cannot agree on.

Hard

5.7.8

Authentication Credentials Invalid

The SMTP authentication credentials (username and/or password) you provided are incorrect or have expired. The server rejected your login attempt. Update your credentials immediately.

Hard

5.7.13

User Account Disabled

The account authenticated successfully and was then refused because an administrator has disabled it. Retrying and re-entering the password cannot help; the block is administrative and only an administrator can lift it.

Hard

5.7.14

Trust Relationship Required

The submission server will not accept this message without a configured trust relationship with a third party. In practice most people meet this as Gmail asking you to sign in through a browser before an application is allowed to send.

Hard

5.7.20

No Passing DKIM Signature Found

The receiving server required a valid DKIM signature and your message did not carry one that passed. Either nothing signed the message, or a signature was present and failed verification.

Hard

5.7.21

No Acceptable DKIM Signature Found

The message carried DKIM signatures that verified correctly, and the receiver rejected them anyway because none met its policy. Passing is not the same as being acceptable.

Hard

5.7.22

No Valid Author-Matched DKIM Signature Found

DKIM passed, but the signing domain does not match the author address in the From header. This is a DKIM alignment failure stated in its own code rather than folded into a DMARC result.

Hard

5.7.23

SPF Validation Failed

Your message completed an SPF check that returned a fail, and the receiving server rejects on that result. The sending IP address is not authorised to send for the domain in the envelope sender.

Hard

5.7.24

SPF Validation Error

SPF evaluation could not be completed. This is not a fail, it is a broken record or a DNS problem that stopped the check from producing an answer at all, and many receivers treat that as a fail.

Hard

5.7.32

From Header Not Aligned with SPF or DKIM

SPF or DKIM passed, but for a different domain than the one in the visible From header. Alignment is what DMARC actually tests, and passing the underlying check is not enough on its own.

Hard

5.7.40

DMARC Record Missing or Invalid

Google rejected the message because the sending domain publishes no DMARC record, or publishes one it cannot parse. Since February 2024 a valid DMARC policy is a requirement for bulk senders, not a recommendation.

Hard

5.7.57

Client Not Authenticated to Send Mail

Exchange Online received your message on the submission port without authentication. This is the error almost every application hits the first time it tries to send through Microsoft 365.

Hard

5.7.321

STARTTLS Not Supported by Destination

The destination mail server does not offer STARTTLS, and your policy requires encryption, so the message was not sent in the clear. The fix belongs to the receiving domain.

Hard

5.7.322

Destination Certificate Expired

The receiving server presented an expired TLS certificate and enforcement refused to proceed. Nothing at your end caused it and nothing at your end can fix it.

Hard

5.7.323

DANE Validation Failed

The destination domain publishes DANE records and the certificate presented did not match them. DANE is stricter than ordinary TLS and offers no fallback.

Hard

5.7.324

Destination DNSSEC Records Invalid

The destination domain returned DNSSEC records that failed validation. Because DANE depends on DNSSEC, a broken chain makes the certificate binding untrustworthy and delivery stops.

Hard

5.7.325

Certificate Hostname Mismatch

The destination presented a valid, unexpired certificate that does not cover the hostname it was reached at. Under enforcement, a name mismatch is as fatal as an expired certificate.

Hard

5.7.25

Reverse DNS Validation Failed

Your sending IP address does not have a valid reverse DNS (PTR) record, or the PTR record does not match the forward DNS. Google requires all senders to have valid forward-confirmed reverse DNS (FCrDNS) on their sending IPs.

Hard

5.7.26

DMARC Authentication Failed

Your email failed DMARC authentication and the domain owner's DMARC policy specifies rejection. Both SPF and DKIM either failed or were not aligned with the From domain. This is one of the most important errors to fix immediately.

Hard

5.7.27

Sender Address Has Null MX

Your sending domain has published a null MX record, declaring it does not accept email. Since it cannot receive bounces or replies, many receiving servers reject email from domains with null MX. Fix your DNS records immediately.

Hard

5.7.28

Mail Flood Detected

The receiving server has detected an abnormally high volume of email from your IP or domain and is rejecting messages as a flood protection measure. This indicates a possible compromised server, spam run, or severely excessive sending volume.

Hard

5.7.29

ARC Validation Failed

The ARC (Authenticated Received Chain) validation failed for this message. ARC is used to preserve authentication results through email forwarding. A failed ARC chain indicates the message was improperly modified or the ARC signatures are invalid.

Hard

5.5.3

Too Many Recipients

Your email attempted to include more recipients than the receiving server allows in a single message. Most servers limit recipients per message to 50-500. Split large recipient lists across multiple messages.

Hard

5.5.0

Other or Undefined Protocol Status

Something went wrong in the SMTP conversation itself rather than with the message or the recipient. The receiver has not narrowed it further, so the reply text and your own session log are the diagnosis.

Hard

5.5.1

Invalid Command

A command was issued that the server does not implement, or was issued out of sequence. Gmail returns it for at least seven distinct situations, most of them a client speaking SMTP incorrectly.

Hard

5.5.2

Syntax Error

The server received a command it could not parse. Usually a malformed address, a stray character, or a line that broke the encoding the protocol expects.

Hard

5.5.4

Invalid Command Arguments

A valid SMTP command carried invalid arguments. Gmail returns it most often for a HELO or EHLO greeting that is empty or not a proper hostname.

Hard

5.6.0

Malformed Message or Media Error

The message itself is malformed. Google returns it for mail that is not RFC 5322 compliant, which usually means broken headers or an encoding the receiver cannot process.

Hard

5.3.4

Message Too Big for System

Your message is too large for the receiving mail system to process, even if the individual mailbox could accept it. This is a system-wide size limit, not a per-mailbox limit. Reduce the message size.

Hard

5.4.4

Unable to Route

The mail system could not work out where to send the message next. Almost always a DNS problem: the recipient domain has no usable MX record, or the records point somewhere unreachable.

Hard

5.4.6

Routing Loop Detected

The message was forwarded between servers too many times and was stopped to prevent an endless loop. Two systems are each convinced the other should deliver it.

Hard

5.4.8

MTA-STS Validation Failed

The destination publishes an MTA-STS policy and its own mail servers do not satisfy it. The domain has promised a security guarantee its infrastructure is not delivering.

Hard

5.4.5

Daily Sending Limit Exceeded

The sending account has exhausted its daily quota. Google numbers it 5.x, but the limit resets on a rolling 24-hour window, so this is a wait rather than a permanent failure and the address must not be suppressed.

Soft

5.4.1

Relay Access Denied

The receiving server will not relay your message or has refused the recipient outright. In Exchange Online this is one of the most common permanent rejections senders meet.

Hard

5.7.30

REQUIRETLS Support Required

The message requires mandatory TLS encryption for delivery (REQUIRETLS), but the next hop in the delivery chain does not support it. The message cannot be delivered without guaranteed encryption.

Hard