What is the difference between SMTP 4xx and 5xx codes?

A 4xx reply is a transient (temporary) failure — the mailbox is busy, storage is low, or the server is overloaded — so the sender should queue the message and retry later. A 5xx reply is a permanent failure such as no such user or a policy rejection, and the sender should bounce the message rather than retry.

What does SMTP 550 mean?

550 means the requested mailbox is unavailable. In practice it usually signals no such user, a non-existent address, or a message rejected by the recipient server's policy (spam, blocklist, or relaying denied). It is a permanent failure, so retrying with the same address will not help.

Why did I get a 421 reply?

421 means the service is not available and the connection is closing — typically the server is shutting down, overloaded, or throttling your IP. It is transient, so a well-behaved sender waits and retries the whole transaction later rather than treating it as a permanent bounce.

What are enhanced status codes?

Enhanced status codes (RFC 3463) are dotted triplets like 5.7.1 that follow the three-digit reply. The first digit mirrors the class (2 success, 4 transient, 5 permanent), the second is the subject (addressing, mailbox, security) and the third gives detail. They give software a precise, machine-readable reason beyond the human text.

Is this SMTP reference private?

Yes. The reply-code table, search and filtering all run locally in your browser with no account and no network requests. Nothing you type is ever sent anywhere.

What is the SMTP Reply Codes Reference?

Searchable SMTP reply code reference from RFC 5321. Look up any three-digit code, see its class, plain-English meaning, whether to retry, and the matching RFC 3463 enhanced status code. It runs free in your browser on Gera Tools, with nothing uploaded.

SMTP Reply Codes Reference

Name: SMTP Reply Codes Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

The SMTP Reply Codes Reference is a searchable lookup for every three-digit reply an SMTP server can send, from the 220 greeting to a 550 rejection. If you have ever read a bounce message or a mail log and wondered whether to retry or give up, this tool answers it: each code shows its class, plain-English meaning, retry guidance, and the matching RFC 3463 enhanced status code.

How it works

SMTP, defined in RFC 5321, uses a three-digit reply code on every server response. The first digit is the whole story for delivery decisions:

2yz  Positive completion   — the command succeeded
3yz  Positive intermediate — server needs more input (DATA, AUTH)
4yz  Transient negative    — temporary failure, retry later
5yz  Permanent negative    — hard failure, do not retry

The second digit groups by subject (syntax, information, connections, mail system) and the third adds specifics. A sending mail server inspects only the first digit to decide whether to deliver, requeue or bounce. This tool keeps the full table in your browser and filters it instantly by number, text or enhanced code.

A full delivery session, annotated

S: 220 mail.example.com ESMTP ready
C: EHLO sender.example.org
S: 250-mail.example.com Hello
S: 250 AUTH PLAIN LOGIN
C: MAIL FROM:<[email protected]>
S: 250 OK
C: RCPT TO:<[email protected]>
S: 250 Accepted
C: DATA
S: 354 Start input; end with <CRLF>.<CRLF>
C: (message headers and body)
C: .
S: 250 OK: queued as 12345
C: QUIT
S: 221 Closing connection

Every server response has a three-digit code. The 354 is the intermediate prompt — the server is telling you to send the message body. Terminate the body with a line containing only a . and the server returns a final 250 if it accepted the message into its queue. Note that 250 OK: queued means accepted for delivery, not delivered — a subsequent DSN or bounce may still arrive.

Common bounce codes and what to do

Code	Enhanced	Meaning	Action
421	4.3.0	Service temporarily unavailable	Wait 30+ min, retry
450	4.2.1	Mailbox busy or temporarily locked	Retry with backoff
451	4.3.5	Server processing error	Retry; contact receiving postmaster if persistent
452	4.5.3	Mailbox full or storage exceeded	Retry later
550	5.1.1	No such user / mailbox unavailable	Remove from list; do not retry
550	5.7.1	Policy rejection (spam, blocklist)	Review sending practices; check blocklists
551	5.1.6	User not local — forwarded	Check the referral address in the reply text
552	5.3.4	Message too large	Reduce message size
553	5.1.3	Bad address syntax	Fix the sender or recipient address
554	5.7.0	Transaction failed	Read the trailing text for the exact reason

Enhanced status codes — reading the triplet

Enhanced codes (RFC 3463) follow the three-digit SMTP reply and have the form X.Y.Z. The first digit mirrors the SMTP class (2 = success, 4 = transient, 5 = permanent). The second digit is the subject:

0 = other or undefined
1 = addressing
2 = mailbox status
3 = mail system
4 = network and routing
5 = mail delivery
6 = message content
7 = security or policy

The third digit refines further. So 5.2.2 is a permanent failure about mailbox status, specifically quota exceeded — very different from 5.7.1, which is a security/policy rejection. When your bounce message shows a generic 550, reading the enhanced code tells you which of many possible 550 situations you are dealing with.