API reference

The endpoint pages under this section are generated from the API's own route definitions at build time, so they always reflect the code. The raw spec is at /openapi.json (OpenAPI 3.1).

Base URL

Authentication

Every endpoint (except the SES event ingestion webhook) requires an API key created in the dashboard:

Authorization: Bearer ms_...

Keys have a permission level: full access keys can use every endpoint, sending only keys are confined to /emails* (anything else returns 403 restricted_api_key) — and even there, GET /emails, GET /emails/{id} and DELETE /emails/{id} need full access, since reads return stored bodies and the team's whole archive. A key can additionally be scoped to a single domain, restricting which from addresses it may send.

Resend compatibility

Request and response shapes match Resend's API, so official Resend SDKs work against MillionSend by pointing their base URL at it. CI runs the official resend npm package against every endpoint as a conformance gate. The few remaining deltas are deliberate and loud:

• Attachments take inline base64 content only — a remote path URL is rejected with 422 (never fetched), as is content_id (inline images).
• Contacts are team-global — audience endpoints are served as aliases of segments, and contact endpoints work with or without an audience id (see Contacts).
• POST /domains takes an optional region, which must be one of the SES regions the deployment serves — the values its request schema lists, the first being the default — and is rejected with 422 otherwise. A domain has one region: to move it, delete it and add it again.
• Broadcasts support canceled, a status outside Resend's union, and broadcast sends, POST /emails and POST /emails/batch can return 403 sending_paused when your bounce or complaint rate crosses the SES enforcement thresholds. Broadcast sends alone can also return 403 broadcasts_paused while the platform's aggregate rate in the sender's SES region is recovering; it is per region, leaves transactional email untouched, and clears on its own.
• A broadcast send answers with finishes_at (the estimated instant the last email goes out, or null), estimated: true and, when the audience exceeds the capacity available now, a warning (paced or queued_behind, with days and a message). Broadcast reads carry sent_count and a live finishes_at; a cancel answers with canceled_remaining. An audience that needs more than 24 days of capacity is refused with 422 broadcast_too_large. See Broadcasts.
• POST /emails and POST /emails/batch return 429 daily_quota_exceeded on a daily plan (Free, Starter) when the day's sending quota is spent and the queued backlog is full — retry after the UTC day rolls over — and 429 monthly_quota_exceeded on a monthly plan (Pro, Scale) at its included volume with overage off; turn on overage in Billing or wait for the period to renew (the message names the date). A batch is accepted or refused whole.
• POST /contacts, POST /contacts/batch and the audience alias return 403 plan_limit_reached when a new contact would take the team past its plan's contact cap (1,000 on Free; paid plans are unlimited). Existing contacts still update; in a batch only the new ones fail.
• GET /usage exists (Resend has no usage endpoint): the effective plan, its limits (emails_per_day on daily plans, emails_per_month on monthly ones, domains, contacts), today's accepted count and, on a monthly plan, a period object — emails_sent, included, overage_enabled, overage_usd_per_1k, starts_at, ends_at. Self-hosted instances report cloud: false with null plan, limits and period.
• DELETE /emails/{id} exists (Resend has no email deletion).
• Custom headers are allowlisted: any X-* name (except X-SES-* and X-MillionSend-*) plus In-Reply-To, References, Importance, Priority, Comments, Keywords, Organization, and the one-click unsubscribe pair — List-Unsubscribe (one or more <https://…> or <mailto:…> targets) with List-Unsubscribe-Post (List-Unsubscribe=One-Click); anything else is a 422. The two must come together, and List-Unsubscribe needs an https target. On a send with a topic_id, a pair you supply replaces the generated one: one-click requests then reach your endpoint, MillionSend records no opt-out for them, and an {{{UNSUBSCRIBE_URL}}} placeholder in the body still resolves to MillionSend's page.
• A send whose every to recipient is on the suppression list or opted out of the topic_id is refused with 422 all_recipients_suppressed (message All recipients are suppressed); recipients dropped from a send that still has someone left are simply omitted. Unsubscribe-origin entries count only when the send has a topic_id; bounce, complaint and manual entries always.
• to, cc and bcc together cannot exceed 50 recipients, and each address must be a single mailbox — a display name containing @ is rejected, and accepted addresses read back in canonical Name <user@host> form.
• Anything unsupported is rejected with 422 rather than silently dropped (e.g. tls on domain update).

Errors

Errors use Resend's format:

{ "statusCode": 422, "name": "validation_error", "message": "..." }

Idempotency

POST /emails and POST /emails/batch accept an Idempotency-Key header. Retrying with the same key and payload returns the original response instead of sending again; the same key with a different payload returns 409.

Pagination

List endpoints accept limit (1–100, default 20) plus after / before cursors carrying an item id from a previous page. Responses include has_more.