Email
SMTP Bounce Code Lookup
Paste or type any SMTP reply code (550, 421, 554...), enhanced status code (5.7.1, 4.7.0...), or keyword from a bounce message to instantly see what it means. Covers RFC 5321 basic reply codes, RFC 3463 enhanced status codes, RFC 7372 authentication-specific codes, and the most common paired codes used by Gmail, Microsoft 365, and other major providers.
94 of 94 codes
5xx — Permanent Failure
| Code | Description |
|---|---|
| 500 | Syntax error — command unrecognized Unknown SMTP verb or malformed command |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 503 | Bad sequence of commands e.g. RCPT before MAIL |
| 504 | Command parameter not implemented |
| 521 | Server does not accept mail |
| 530 | Authentication required |
| 535 | Authentication credentials invalid |
| 550 | Mailbox unavailable / user unknown Most common permanent rejection — user does not exist or policy rejection |
| 551 | User not local — no forwarding address |
| 552 | Exceeded storage allocation Mailbox full |
| 553 | Mailbox name not allowed Invalid recipient address format |
| 554 | Transaction failed / no valid recipients Often combined with 5.7.x for policy rejections |
| 555 | MAIL/RCPT parameter not recognized or implemented |
| 5.0.0 | Other undefined status (permanent failure) |
| 5.1.1 | Bad destination mailbox address User does not exist — most common "user unknown" code |
| 5.1.2 | Bad destination system address Invalid domain |
| 5.1.3 | Bad destination mailbox address syntax |
| 5.1.6 | Destination mailbox has moved, no forwarding address |
| 5.1.10 | Recipient address has null MX Domain publishes "null MX" indicating it does not accept mail |
| 5.2.0 | Other or undefined mailbox status |
| 5.2.1 | Mailbox disabled, not accepting messages |
| 5.2.2 | Mailbox full (permanent) |
| 5.2.3 | Message too large for mailbox |
| 5.2.4 | Mailing list expansion issue |
| 5.3.0 | Other or undefined mail system status |
| 5.3.4 | Message too big for system |
| 5.4.1 | No answer from host (permanent) |
| 5.4.4 | Unable to route — DNS lookup failure |
| 5.5.0 | Other or undefined protocol status |
| 5.5.1 | Invalid command |
| 5.5.2 | Syntax error |
| 5.6.0 | Other or undefined media error |
| 5.6.1 | Media not supported |
| 5.7.0 | Other or undefined security status |
| 5.7.1 | Delivery not authorized, message refused Rejected as spam; DMARC/SPF/DKIM/policy failure |
| 5.7.2 | Mailing list expansion prohibited |
| 5.7.8 | Authentication credentials invalid |
| 5.7.9 | Message authentication required |
| 5.7.10 | Encryption needed |
| 5.7.11 | Encryption required for requested authentication mechanism |
| 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 Sending IP not authorized by the sender domain's SPF record |
| 5.7.24 | SPF validation error |
| 5.7.25 | Reverse DNS validation failed Sending IP lacks valid PTR / rDNS mismatch |
| 5.7.26 | Multiple authentication checks failed |
| 550 5.1.1 | User unknown Recipient mailbox does not exist on destination server |
| 550 5.2.1 | Mailbox disabled Recipient account is suspended or disabled |
| 550 5.2.2 | Mailbox full Recipient over quota — cannot accept more mail |
| 550 5.7.1 | Rejected as spam or by policy DMARC policy rejection, blacklist hit, or content filter |
| 550 5.7.26 | Multiple authentication checks failed Gmail-style rejection: SPF, DKIM, and DMARC all fail |
| 551 5.7.1 | Forwarding denied by policy |
| 552 5.2.3 | Message size exceeds recipient limit |
| 553 5.1.3 | Malformed address / bad recipient syntax |
| 554 5.7.1 | Transaction rejected by policy Your sending IP is blocked or untrusted |
| 554 5.7.9 | Authentication required Server requires SPF/DKIM/DMARC alignment before accepting |
4xx — Temporary Failure
| Code | Description |
|---|---|
| 421 | Service not available — try again later Server shutting down, overloaded, or reputation-gated |
| 450 | Mailbox unavailable (temporary) Mailbox busy or temporarily locked |
| 451 | Requested action aborted — local error Transient server error, retry |
| 452 | Insufficient system storage Mail server out of disk space |
| 454 | Temporary authentication failure |
| 455 | Server unable to accommodate parameters |
| 4.2.1 | Mailbox disabled, not accepting messages |
| 4.2.2 | Mailbox full (temporary) |
| 4.3.0 | Other or undefined mail system status |
| 4.3.1 | Mail system full — out of storage |
| 4.3.2 | System not accepting network messages |
| 4.4.1 | No answer from host |
| 4.4.2 | Bad connection / connection lost |
| 4.4.5 | Mail system congestion |
| 4.4.7 | Message expired in queue Too many retries over too long a period |
| 4.7.0 | Other or undefined security status (transient) Common throttling / greylisting response |
| 4.7.1 | Delivery not authorized, message refused (transient) |
| 421 4.7.0 | Try again later — throttling Gmail-style temporary deferral for suspicious reputation |
| 421 4.7.28 | IP sending rate exceeded Gmail temporary rate limit |
| 451 4.7.1 | Greylisting — retry in a few minutes Server deliberately defers first-time senders |
| 452 4.2.2 | Mailbox full (temporary) |
| 452 4.5.3 | Too many recipients in one message |
2xx — Success
| Code | Description |
|---|---|
| 211 | System status / help reply |
| 214 | Help message Response to a HELP command |
| 220 | Service ready (SMTP greeting banner) |
| 221 | Service closing transmission channel Response to QUIT |
| 235 | Authentication successful Response to AUTH command |
| 250 | Requested action completed Most common success: HELO/MAIL/RCPT/DATA accepted |
| 251 | User not local — will forward Message forwarded to another address |
| 252 | Cannot VRFY user but will accept message |
| 334 | AUTH continuation (server challenge) |
| 354 | Start mail input — end with <CRLF>.<CRLF> Response to DATA command |
| 2.0.0 | Other undefined status (success) |
| 2.1.5 | Destination address valid |
| 2.6.0 | Other or undefined media error (success) |
| 2.7.0 | Other or undefined security status (success) |