The Master List of Gmail SMTP Response Codes

When sending email through Google’s SMTP servers (smtp.gmail.com or smtp-relay.gmail.com), every transaction produces an operational status code defined by RFC 821, RFC 2821, and RFC 3463. These codes tell you whether your message was accepted, temporarily delayed, or permanently rejected.

       ┌── 2xx: Success / Command Acknowledged
       │
SMTP ──┼── 4xx: Transient (Temporary) Failure ── Try again later
Code   │
       └── 5xx: Permanent Failure ────────────── Immediate termination

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2xx Series: Success and Status Codes

These responses confirm that an individual command or overall transmission succeeded.

Code	Enhanced Status	Meaning / Server Response	Diagnostic Context
220	2.0.0	smtp.gmail.com ESMTP ready	Initial connection established successfully.
250	2.0.0	OK or 250-STARTTLS	The requested command completed cleanly. The server is ready for the next instruction.
354	2.0.0	Go ahead / Start mail input	Acknowledges the DATA command. The server is ready to receive the email payload, ending with <CRLF>.<CRLF>.

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4xx Series: Temporary Failures (Transient Rejections)

A 4xx code indicates a temporary delivery obstacle. The sending client or server should hold the message in its queue and retry later.

Code	Enhanced Status	Meaning / Server Response	Root Cause & Remediation
421	4.7.0	Try again later, closing connection	The sending IP address has exceeded Google’s short-term concurrency limits. Throttle connection threads.
421	4.7.28	Gmail has detected an unusual rate of unsolicited mail	The sending IP is temporarily rate-limited due to a sudden spike in spam-like volume. Slow down delivery rates.
450	4.2.1	The user you are trying to contact is receiving mail at a rate that prevents additional messages	The recipient’s mailbox is receiving too many messages per minute. Wait and retry delivery.
451	4.3.0	Mail server has temporarily rejected this message	Google back-end systems experienced a transient operational error. Resubmit the transaction.
452	4.2.2	The email account that you tried to reach has exceeded its storage quota	The recipient’s Google Drive, Gmail, and Google Photos storage is completely full. The message cannot be accepted until they free up space.

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5xx Series: Permanent Failures (Hard Rejections)

A 5xx code indicates a fatal error. The receiving server has rejected the message, and the sending client must terminate the delivery attempt and generate a non-delivery report (NDR/bounce-back).

Authentication and Protocol Failures (5.0.0 – 5.5.9)

Code	Enhanced Status	Meaning / Server Response	Root Cause & Remediation
501	5.5.2	Cannot Decode response / Syntax error	Malformed base64 strings during authentication or invalid syntax in HELO/MAIL FROM: commands.
502	5.5.1	Unimplemented command	The client sent an unsupported or unrecognized command to the server.
503	5.5.1	Bad sequence of commands	Commands were sent out of order (e.g., sending DATA before defining RCPT TO:).
530	5.7.0	Must issue a STARTTLS command first	The client attempted to transmit data or credentials over an unencrypted connection. For step-by-step remediation, see SMTP Error “530 5.7.0 Must issue STARTTLS”.
535	5.7.8	Username and Password not accepted	Invalid credentials, use of legacy basic auth without an App Password, or expired session tokens. Follow the resolution steps in “535 5.7.8” (Authentication Failed).

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Delivery, Storage, and Addressing Failures (5.1.0 – 5.4.9)

Code	Enhanced Status	Meaning / Server Response	Root Cause & Remediation
550	5.1.1	The email account that you tried to reach does not exist	The recipient username was deleted, misspelled, or does not exist on the target domain.
550	5.2.1	The email account that you tried to reach is disabled	The destination account has been suspended or administrative access has been restricted.
550	5.4.5	Daily user sending limit exceeded	The sending account passed its 24-hour limit (500 recipients/day for consumer accounts; 2,000/day for Google Workspace).
552	5.2.3	Your message exceeded Google's message size limits	The message payload (including base64 attachment overhead) exceeds Google’s hard 25 MB inbound/outbound limit.

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Security, Policy, and Reputation Rejections (5.7.0 – 5.7.28)

Code	Enhanced Status	Meaning / Server Response	Root Cause & Remediation
550	5.7.1	Relaying denied or Unauthenticated email is not accepted	The client tried to forward mail to an outside recipient without valid authentication, or the sending domain lacks proper SPF records. See Troubleshooting SMTP Relay: “550 5.7.1 Relaying Denied”.
550	5.7.25	Sending IP does not pass reverse DNS check	The sending server’s public IP lacks an explicit reverse DNS (PTR) record matching its hostname. Configure valid PTR records in your ISP/hosting control panel.
550	5.7.26	This message fails authentication checks (DMARC/SPF/DKIM)	The sending domain failed DMARC policy validation with alignment enforcement (p=reject or p=quarantine). Ensure your DNS TXT records are aligned using How to Fix Gmail Error 550 5.7.26: SPF Missing/Invalid.
554	5.7.0	Message rejected: IP address is on a blocklist	The sending server’s IP address appears on an industry Real-time Blackhole List (RBL), such as Spamhaus or SORBS. Request delisting from the respective list operator.