Reference
SMTP Error Codes Reference
Every SMTP bounce code your cold email might trigger — what it means, why it happens, and how to fix it. Each page covers the receiving server's reasoning, the diagnostic steps that resolve it, and the infrastructure changes that prevent it from coming back.
5xx — Permanent failures (43)
Mail server is saying the message will not be delivered. Most cold email pain lives here — content filters, reputation rejections, policy blocks.
500 Syntax error, command unrecognized
Receiver couldn't parse your SMTP command — protocol error
501 Syntax error in parameters or arguments
Receiver recognized the SMTP command but rejected its arguments
502 Command not implemented
Receiver doesn't implement this SMTP command
503 Bad sequence of commands
SMTP commands sent in the wrong order
504 Command parameter not implemented
SMTP command is supported, but the parameter you supplied isn't
521 Server does not accept mail
Receiver explicitly does not accept inbound mail
530 Authentication required
Receiver requires SMTP authentication before accepting mail
535 Authentication failed
SMTP credentials were rejected — wrong username or password
550 4.4.7
Exchange: message expired — retry timeout reached
550 5.1.0
Office 365: sender address rejected — sender policy violation
550 5.1.1
User unknown — recipient address doesn't exist at this domain
550 5.1.10
Microsoft 365: RESOLVER.ADR.RecipientNotFound — recipient address doesn't exist
550 5.2.2
Gmail: recipient is over quota
550 5.4.5
Gmail: daily sending quota exceeded
550 5.4.6
Gmail: routing loop detected — message bouncing between servers
550 5.7.1
Gmail: daily SMTP relay limit exceeded
550 5.7.1
Gmail: DKIM signature verification failed
550 5.7.1
Gmail: DMARC policy reject — message failed alignment
550 5.7.1
Gmail: our system detected an illegal attachment
550 5.7.1
Gmail: message identified as spam
550 5.7.1
Gmail: no reverse DNS (PTR) — sending IP can't resolve to a hostname
550 5.7.1
Spam policy violation — receiver rejected the message on policy grounds
550 5.7.1
Gmail: SPF hard-fail — sending IP not authorized by SPF record
550 5.7.1
SPF PermError — too many DNS lookups in SPF record
550 5.7.1
Outlook / Microsoft 365: unable to relay
550 5.7.1 [internal]
Gmail: our system has detected an unusual rate of unsolicited mail
550 5.7.9
Message not RFC 5322 / 2822 compliant — rejected for malformed headers
550 5.7.26
Gmail: unauthenticated email — SPF, DKIM, or DMARC failed
550 5.7.28
Gmail: mail rate limit exceeded
550 5.7.28
Gmail: IP has too many ratings — high complaint or spam-folder rate
550 5.7.501
Outlook / Microsoft 365: access denied — banned sender
550 5.7.520
Outlook / Microsoft 365: message blocked by content policy
550 Mailbox unavailable
Catch-all permanent rejection — the most common 5xx code
551 User not local
Recipient isn't local to this server — try the forwarding address
552 5.2.3
Gmail: message exceeded maximum size
552 5.3.4
Message size exceeds the destination's policy limit
552 Mailbox quota exceeded
Recipient mailbox is full — over its storage quota
553
553: From address invalid — sender mailbox name not allowed
553 Mailbox name invalid
Mailbox name not allowed — syntactically rejected by receiver
554 5.7.1
Microsoft 365: blocked by Exchange Online Protection content filter
554 5.7.1
Sender not authorized — recipient requires pre-approval
554 5.7.1
Message permanently rejected as spam
554 Transaction failed
Generic transaction failure — receiver rejected the message, cause varies
4xx — Temporary failures (17)
Mail server is deferring — try later. Often a transient rate-limit, grey-listing, or short-term reputation hold. Still worth fixing if they're recurring.
421 4.4.5
Gmail: server busy — too many concurrent connections
421 4.7.0
Gmail: IP not in whitelist for RCPT domain
421 4.7.0
Mail rejected — sending IP is Spamhaus-listed or has reputation issues
421 4.7.1
Gmail: message rate limit exceeded — sender deferred
421 4.7.28
Gmail: too many messages from your IP
421 4.7.30
Gmail: content not allowed — message deferred on policy
421 4.7.32
Gmail: suspicious account activity — sending account flagged
421 4.7.500
Outlook / Microsoft 365: server busy — temporary throttle
421 Service not available
Service not available — receiving server is busy or closing the connection
421-4.7.0
Gmail: messages rejected due to suspicious content
450 4.2.1
Mailbox temporarily disabled — recipient inbox not accepting mail
450 Mailbox temporarily unavailable
Requested mailbox unavailable — temporary lock or greylisting
451 4.7.0
Microsoft 365: temporary server error — transient policy defer
451 4.7.500
Microsoft 365: server busy — transient throttling
451 4.7.650
SPF record syntax or evaluation error — temporarily deferred
451 Local error
Local error in processing — receiving server hit an internal problem
452 Insufficient system storage
Receiving server has insufficient storage to accept the message
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