Reference · 12 codes

SMTP Bounce Code Reference

What each SMTP reply code and RFC 3463 enhanced status code actually means, what typically causes it, and whether to retry or suppress the address.

Permanent Failures (5xx)

Definitive rejections — suppress the address.

550 5.1.1

User unknown / mailbox does not exist
550 5.7.1

Message rejected due to policy (spam/content)
551 5.1.6

Mailbox has moved, no forwarding address
552 5.2.2

Mailbox full (permanent quota failure)
553 5.1.3

Invalid mailbox name syntax
554 5.7.1

Transaction failed (blocked by policy/reputation)
554 5.4.6

Routing loop detected (too many hops)

Temporary Failures (4xx)

Transient issues — retry before giving up.

421 4.7.0

Service not available / connection rejected (policy)
450 4.2.0

Mailbox temporarily unavailable
451 4.3.0

Local error in processing / greylisting
452 4.2.2

Mailbox full (over quota, temporary)

Success (2xx)

The server accepted the address.

250 2.1.5

Recipient address OK

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