Queues one email. The response is immediate; delivery happens in the background and is reported through GET /transactional/emails/{id} and your webhook.

Idempotency. Send an Idempotency-Key header (1 to 256 characters) to make retries safe. Within 24 hours, the same key with the same body returns the original message (with an idempotent-replayed: true header); the same key with a different body is a 409 invalid_idempotent_request.

Sandbox. Send from onboarding@sendfoxmail.com with no DNS setup. It delivers only to your own account email and to the four test addresses. Test addresses never reach a mailbox and never count toward your limits; each one synthesizes an outcome so you can exercise your integration: delivered@sendfoxmail.com, bounced@sendfoxmail.com, complained@sendfoxmail.com, suppressed@sendfoxmail.com.

Limits. Transactional email has no quota of its own: every address in to, cc and bcc counts as one send against your SendFox plan, the same monthly sends your campaigns use, with no daily cap. When the plan has a cap, every response carries x-sendfox-sends-remaining, the sends left across campaigns and transactional email; a plan without an enforced cap sends no header. Past the cap, sends are refused with 429 monthly_quota_exceeded carrying remaining and requested. Each message holds at most 10 addresses across to, cc and bcc, at most 2 in bcc. Requests are limited to 5 per second and 300 per minute per key.

Custom headers. Pass headers to add list-management and threading headers. For one-click unsubscribe (RFC 8058, which Gmail and Yahoo require of bulk senders) send List-Unsubscribe with an https: URL in angle brackets and List-Unsubscribe-Post set to List-Unsubscribe=One-Click; the URL must accept a POST whose body is List-Unsubscribe=One-Click and unsubscribe the recipient without any further click. See the headers field for what is allowed.

{
  "from": "Acme <hello@mail.acme.com>",
  "to": [
    "jane@example.com"
  ],
  "subject": "Your login code",
  "html": "<p>Your code is <strong>483 920</strong>.</p>",
  "tag": "login_code",
  "headers": {
    "List-Unsubscribe": "<https://acme.com/unsubscribe?u=4f0c>, <mailto:unsub@acme.com>",
    "List-Unsubscribe-Post": "List-Unsubscribe=One-Click"
  },
  "attachments": [
    {
      "filename": "invite.ics",
      "content_type": "text/calendar; method=REQUEST"
    }
  ]
}

{
  "id": "tm_4f0c9b2e7d1a5c3e8b6f2a9d",
  "status": "queued"
}