> ## Documentation Index > Fetch the complete documentation index at: https://mintfax.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # FAQ > Answers to the questions developers ask most often about the mintfax API ## Can I receive faxes through mintfax? No. mintfax is an outbound fax API. You send faxes through the API; there is no inbound fax number or receive endpoint. ## Do I need a fax number to send? No. You provide the destination number and the document, and mintfax handles delivery. No dedicated fax number or hardware required. ## What file formats can I send? PDF, DOCX, XLSX, JPG, PNG, TIFF, HTML, and TXT. Maximum file size is 10 MB. PDF is recommended for the most predictable rendering. See [file types and sizes](/docs/file-types) for conversion notes and edge cases. ## How long does a fax take to deliver? Most faxes complete in 1-5 minutes depending on page count and destination line quality. The analog phone leg is the bottleneck, not the API. See [time-sensitive documents](/docs/time-sensitive) for tips on optimizing delivery speed. ## How do I know if a fax was delivered? Poll `GET /fax/{id}` for the status, or register a webhook endpoint to receive `fax.delivered` and `fax.failed` events in real time. See [track delivery](/docs/track-delivery) for both approaches with code samples. ## What happens if the destination line is busy? mintfax retries automatically. The default is 3 attempts, configurable up to 5 per fax. Each delivery attempt fires a `fax.sending` webhook event. If every attempt fails or a terminal error code is returned, the fax moves to `failed` and the credit hold is released. ## Can I resend a failed fax? Yes. `POST /fax/{id}/resend` resubmits the original document as a new fax. A new credit hold is placed. The original document must still be within the [retention window](/docs/data-retention). ## Are there ads or branding on the faxes I send? No. Faxes contain only your document and optional cover page. mintfax does not add advertisements, logos, or branding of any kind. ## How much does a fax cost? Credits are prepaid and charged per page. Check your current balance with `GET /account/balance`. See [credits and billing](/docs/credits) for the hold lifecycle and balance monitoring. ## Are there monthly minimums, setup fees, or contracts? No. There are no monthly fees, no setup costs, and no long-term contracts. You buy credits when you need them and use them at your own pace. Cancel anytime. ## Can my account recharge automatically? Yes. Enable auto top-up in the dashboard to add credits automatically when your balance drops below a threshold you set. The `balance.low` webhook event fires at the same threshold if you want to handle it programmatically. ## What payment methods do you accept? Visa, Mastercard, American Express, and Discover. Payments are processed through Stripe. ## What is the difference between sandbox and live? Two completely isolated environments. Sandbox keys (`mfx_test_`) route to simulated infrastructure with [magic fax numbers](/docs/sandbox) that produce deterministic outcomes. Live keys (`mfx_live_`) send real faxes. The API surface is identical. See [environments](/docs/environments). ## Can I test without sending real faxes? Yes. The [sandbox](/docs/sandbox) provides magic numbers that simulate success, busy, transient failure, permanent failure, and insufficient balance. No real fax is sent and no real money is spent. ## Is there an SDK? A [Node.js SDK](/docs/node) is available. For other languages, call the REST API directly. The API uses standard Bearer auth and multipart/form-data for file uploads. ## How do I handle errors? Every error returns a consistent JSON envelope with a stable `error` code, a human-readable `message`, and an `action` hint. Match on the `error` field, not the HTTP status or message text. See the [error catalog](/docs/errors) for every code and recommended action. ## What happens if my webhook endpoint is down? mintfax retries failed webhook deliveries with exponential backoff (up to four attempts over about 18 minutes total). Your endpoint should respond within 30 seconds; longer than that is treated as a timeout. Deduplicate using the `webhook-id` header since retries deliver the same event more than once. See [Delivery and retries](/docs/webhooks/delivery-and-retries). ## How long is fax data retained? Configurable per environment. Standard mode retains fax data for a configurable period (default 90 days). Zero-footprint mode deletes fax documents immediately after the fax reaches a terminal state. Billing records are retained permanently in both modes. See [data retention](/docs/data-retention). ## Is mintfax HIPAA compliant? Yes. Every paid environment can sign a BAA at no additional cost. Encryption, audit logging, retention controls, and cover-page redaction ship on every plan. See [HIPAA compliance](/docs/hipaa) for the full compliance boundary and [patient health information](/docs/patient-health-info) for the implementation checklist. ## Does mintfax support PCI-DSS? mintfax is not PCI-DSS certified. If your fax documents contain cardholder data, [zero-footprint mode](/docs/data-retention) ensures no document content persists after delivery. See [PCI-DSS](/docs/pci-dss) for what this covers and what it does not.