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WEBHOOK

When it fires

At the start of each delivery attempt against the destination. A fax that retries on a transient failure will fire fax.sending again before the next attempt. The current attempt is recorded in attempts[] with started_at set and completed_at null. Previous attempts (if any) are also present with their full detail.

What your handler should do

  • Use this for in-flight visibility - “fax is currently being sent.”
  • If you display fax status to your users, this is the event that moves the display from “queued” to “in progress.”
  • Do not finalize success or failure on this event. Wait for fax.delivered or fax.failed.
The number of fax.sending events for a single fax depends on how many delivery attempts run, which is bounded by max_attempts (1 to 5, default 3).

Authorizations

Authorization
string
header
required

API key as Bearer token. Use an account key (mfx_acct_...) for /account/* endpoints, and an environment key (mfx_test_... for sandbox, mfx_live_... for live) for /environment/*, /faxes, /webhooks, and /events.

Body

application/json

Event delivery to your subscribed webhook endpoint.

Webhook delivery for fax.sending.

id
string
required

Event identifier (evt_-prefixed). Stable across retry attempts.

Example:

"evt_8aZqRm4yT3vK7pNxJ2bH9c"

type
enum<string>
required

Event type. Always fax.sending for this delivery.

Available options:
fax.sending
created
integer
required

Unix epoch seconds when the event was generated.

data
object
required

Response

2XX

Acknowledged. mintfax treats any 2xx response as a successful delivery.

Last modified on July 14, 2026