# Documentation


- **Get started**
- [Introduction](/docs): Send SMS and transactional email from your app with one HTTP call, and know exactly what happened to every message.
- [Quickstart](/docs/quickstart): Get an API key and send your first SMS in five minutes.
- [Authentication](/docs/authentication): Authenticate every request with an API key in the X-Api-Key header.
- [Sending access](/docs/sending-access): Promotional and transactional traffic, what each account can send, and how to unlock more.

- **Guides**
- [Send one-time codes](/docs/guides/send-otp): Deliver login and verification codes fast, from your own sender ID, without ever sending a code twice.
- [Send bulk SMS](/docs/guides/bulk-sms): Send campaigns and announcements to thousands of recipients, in safe, resumable batches.
- [Send transactional email](/docs/guides/transactional-email): Receipts, alerts and codes by email, with bounces handled for you.
- [Track delivery](/docs/guides/delivery-status): Know whether each message reached its recipient, by webhook or by polling.
- [Phone numbers](/docs/guides/phone-numbers): Which number formats we accept, and how we normalise them.
- [Pricing and segments](/docs/guides/pricing-and-segments): What a send costs, when you're charged, and when you get money back.

- **Webhooks**
- [Webhooks](/docs/webhooks): Get a signed HTTP request the moment a message is delivered, fails or bounces.
- [Events](/docs/webhooks/events): Every event type, when it fires, and exactly what it contains.
- [Verify signatures](/docs/webhooks/verify-signatures): Prove a webhook request came from SoftLinkSMS and wasn't altered or replayed.
- [Retries and the event log](/docs/webhooks/retries-and-event-log): What happens when your endpoint is down, and how to catch up afterwards.

- **Reference**
- [Errors](/docs/reference/errors): One error shape everywhere, a stable code for every case, and what to do about each.
- [Rate limits](/docs/reference/rate-limits): How many requests each account can make, and how to stay within them.
- [Idempotency](/docs/reference/idempotency): Retry any send without sending, or charging, twice.
- [Pagination](/docs/reference/pagination): Walk through long lists with stable cursors.
- [Message lifecycle](/docs/reference/message-lifecycle): Every status a message can have, and what moves it on.
- [Versioning](/docs/reference/versioning): What we may change without notice, and what we never break.
- [Changelog](/docs/reference/changelog): Every change to the API, newest first.