Webhooks

How a sending provider tells your Mimeo what happened to a message it took — delivered, bounced, complained, suppressed.

POST /webhooks/email/:provider

One route per configured adapter. Verification belongs to the adapter, because how a provider proves a request is its own differs per provider.

Verification

Provider	How
Resend	Svix signature headers — `svix-id`, `svix-timestamp`, `svix-signature` — HMAC-SHA256 over `id.timestamp.body`, with a five-minute freshness window. Set the signing secret under Settings → Provider → Credentials.
Postmark	Postmark can't sign, so the claim is its webhook secret in the URL: `?secret=…`, compared in constant time against `provider.postmark.webhook_secret` — set under Settings → Provider → Credentials.
Providers that can't sign	A shared secret in the URL: `?secret=…`, compared in constant time against `provider.webhook_secret`.

A provider that signs does not fall back to the shared secret. A signature is a much stronger claim, and accepting the weaker one from a provider that offers the stronger would undo the point of checking.

Responses

Status	When
`200`	Understood — including an event type we don't act on. Retrying a no-op helps nobody.
`401`	Verification failed.
`404`	No adapter by that name.
`400`	The body isn't parseable. A retry of the same bytes won't fix it.
`500`	Genuinely failed to process. Please retry.

The internal vocabulary

Adapters normalize their provider's own event names into five, and nothing above the adapter ever sees a provider's vocabulary:

Type	Effect
`delivered`	The send is marked delivered.
`bounced`	Recorded with a `bounce_kind`. A hard bounce suppresses the person; a soft one (a full mailbox) does not — that's a try-again, not a dead address.
`complained`	Suppresses the person. Reason `complaint`.
`unsubscribed`	Suppresses the person.
`suppressed`	The provider suppressed them on its side. Suppresses locally too.

A suppression from a webhook is recorded with source provider_webhook, which stops it being pushed straight back to the provider that just reported it.

At-least-once and unordered

Assumed of every provider, whatever they promise. Handling is built for it:

Every write is set-if-not-already. The same event twice leaves exactly what it left the first time — no double timeline entry, no second unsubscribe.
A weaker fact never overwrites a stronger one. A delivered arriving after a bounced for the same message is ignored: a message that bounced doesn't become delivered because a stale event turned up late.
Attribution is by provider message id. Never by address alone — that would attribute an event to whichever message happened to be most recent.

If a provider lacks them

A provider without webhooks would be polled instead. Either way the local database is the authority and the standing guard checks it before every send — a provider without webhooks is slower to reconcile, not less safe.