Errors

One envelope, one code, and what each means.

A failure is always the same shape, with an HTTP status that means the same thing as the code inside it.

json

{
"error": {
  "code": "validation",
  "message": "Enter only the part before the @.",
  "field": "local_part"
}
}

message is written for a person and is safe to show one. It never carries a stack, a DNS response code or a provider's wording — if you are surfacing API errors in your own interface, this is the string to use.

The codes

Code	Status	What it means
`validation`	400	The request is malformed. `field` says where.
`unauthenticated`	401	No key, or a key that has been deleted.
`forbidden`	403	This key may not do that: its access or its reach.
`blocked`	403	A rule on the address refused it. Remove the rule.
`not_permitted`	403	The address's `send_policy` refused it.
`not_found`	404	No such resource — or one that is not yours.
`conflict`	409	It already exists, or something else changed first.
`gone`	410	It was here, and has been deleted.
`resync`	410	Too far behind to catch up: page again from the top
`entitlement`	402	Your plan does not include this.
`abuse_blocked`	403	The account is paused pending review.
`rate_limited`	429	Too many requests. See the headers below.
`unavailable`	503	Ours, and temporary. Retry.
`internal`	500	Ours, and unexpected.

forbidden and not_found deliberately return the same sentence. Confirming that a resource exists but belongs to somebody else is itself a leak, so the API does not.

gone is the one place we do distinguish existed from never existed, and it is scoped so that the distinction tells you nothing you did not already have: you get 410 only for something deleted from a resource you can already read. Asked about somebody else's, it is a 404 like anything else. It is answered for as long as the event log remembers the deletion; after that the honest answer is 404 again, because we no longer know.

Rate limit headers

Every successful response carries them, so a client can pace itself rather than discover the limit by hitting it.

http

X-RateLimit-Limit: 120
X-RateLimit-Remaining: 118
X-RateLimit-Reset: 1757760060

Reset is unix seconds. A 429 also carries Retry-After.

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The codes

Rate limit headers