Email Bounce Code Lookup

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.

Info

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.

Info

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.

Info

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.

Info

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.

Info

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

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

5.1.0

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

5.2.1

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

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

4.3.1

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

4.3.2

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

4.4.7

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

4.7.0

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

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

5.7.5

Authentication Required

The server requires cryptographic authentication that your message failed to provide. This is different from SMTP AUTH - it refers to email authentication mechanisms like SPF, DKIM, or mutual TLS that the receiving server mandates.

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

DKIM Signing Required but Not Present

The receiving server requires messages from your domain to be signed with DKIM, but your message had no DKIM signature. Configure DKIM signing on your sending server to resolve this.

Hard

5.7.14

DKIM Verification Failed

Your email had a DKIM signature but it failed verification. The message was modified in transit, the DKIM key in DNS does not match, or the signature is malformed. This causes authentication failure and potential DMARC rejection.

Hard

5.7.20

SPF Validation Failed

Your email failed SPF (Sender Policy Framework) authentication. The sending IP address is not authorized in your domain's SPF record to send email on behalf of your domain. Update your SPF record to include the sending server.

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

5.3.4

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

5.7.30

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