> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://docs.usglobalmail.com/guides/core-concepts/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://docs.usglobalmail.com/_mcp/server.

# Core concepts

> Official documentation for the USGM API — REST endpoints, SDKs (TypeScript, Python, PHP, Go), an MCP server, and guides for your US Global Mail mailbox.

## Docs

- [Authentication](https://docs.usglobalmail.com/guides/core-concepts/authentication.md): Every request is authenticated with an API key sent as a bearer token.
- [Pagination](https://docs.usglobalmail.com/guides/core-concepts/pagination.md): List endpoints return a page of results and an opaque cursor for the next page.
- [Errors](https://docs.usglobalmail.com/guides/core-concepts/errors.md): Every non-2xx response is an RFC 9457 problem+json document.
- [Rate limits](https://docs.usglobalmail.com/guides/core-concepts/rate-limits.md): Requests are throttled at the edge, keyed by API key.
- [Webhooks](https://docs.usglobalmail.com/guides/core-concepts/webhooks.md): Get a callback the moment mail arrives — or a scan, open request, or shipment changes — instead of polling.
- [System status](https://docs.usglobalmail.com/guides/core-concepts/system-status.md): Check whether the USGM API, MCP server, USGM Core or other system modules are operating normally.
- [Resource models](https://docs.usglobalmail.com/guides/core-concepts/resource-models.md): The objects the API returns, and how they relate.