Reference
SMTP reply codes
What each reply means, which RFC defines it, what the major providers actually say, and what a verifier returns when a mail server sends it.
5xx — permanent failures
The receiving server will not accept this message, and retrying in the same form will produce the same reply. Some of these are about the recipient; some are about you.
550 5.1.1
mailbox does not exist
A 550 5.1.1 reply means the receiving mail server checked the address and the mailbox does not exist.
550 5.1.10
recipient not found
A 550 5.1.10 reply means the receiving system looked the recipient up and did not find it.
553 5.1.3
bad destination mailbox address syntax
A 553 5.1.3 reply means the receiving server could not parse the recipient address at all — it is malformed rather than missing.
550 5.4.1
recipient address rejected, access denied
A 550 5.4.1 reply usually means the receiving system refused the recipient — commonly because the address is not in its directory, or because a policy blocks the sender.
550 5.7.1
delivery not authorized, message refused
A 550 5.7.1 reply means the receiving server refused the message on policy or security grounds.
550 5.7.23
SPF validation failed
A 550 5.7.23 reply means the receiving server checked SPF for the sending domain and the check did not pass.
552 5.2.2
mailbox over quota
A 552 5.2.2 reply means the mailbox exists but has no room for the message.
4xx — transient failures
The condition is temporary and the sender should try again. Writing any of these into a suppression list removes working mailboxes from a list permanently.
450 4.7.1
greylisted, try again later
A 450 4.7.1 reply is a temporary refusal, and on most servers it means greylisting: the receiving server has not seen this sender before and is asking it to come back.
451 4.3.0
temporary local error
A 451 4.3.0 reply means something went wrong in the receiving mail system itself and the message could not be processed now.
452 4.2.2
mailbox full, try again later
A 452 4.2.2 reply means the mailbox exists but has no storage space left, sent as a temporary failure so the sending server queues and retries.
421 4.7.0
service not available, closing connection
A 421 reply means the server is closing the connection rather than continuing — it is too busy, throttling you, or applying a temporary security decision.
2xx — accepted
The server took the recipient. Whether that confirms the mailbox exists depends on something the reply does not say.
The concepts behind these codes — catch-all domains, greylisting, hard bounces, what an unknown verdict is — are defined in the deliverability glossary.
Sources
- RFC 5321 — Simple Mail Transfer Protocol
- RFC 3463 — Enhanced Mail System Status Codes
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes
- IANA — SMTP Enhanced Status Codes registry
Primary sources checked 11 September 2026. If something here is out of date, tell us and we will correct it.
Questions about bounce codes
What are email bounce codes?
The reply codes a receiving mail server returns when it refuses a message: a three-digit SMTP code from RFC 5321 and, usually, an enhanced status code of the form class.subject.detail from RFC 3463. Together they say whether the failure is permanent, which part of the system had the problem, and what it was.
How do I know all possible email bounce codes and reasons?
The authoritative list of enhanced status codes is the IANA registry established by RFC 5248 — that is the complete set, and it is linked below. The three-digit reply codes are defined in RFC 5321. Individual providers add their own wording on top, which is why the reply text matters as much as the digits.
How do SMTP codes work?
Every command in an SMTP conversation gets a three-digit reply. The first digit says success, transient failure or permanent failure; the rest narrows it down. Most servers also send an enhanced status code of the form class.subject.detail, which is registered with IANA and is far more specific than the three digits alone.
See the code before the bounce
Every verification result carries the SMTP code and enhanced code the server actually returned. 100 credits a month, free and renewing.