# E04011 — RecipientDomainNotAllowed

Sending to reserved testing domains such as @example.com or @test.com is not supported. These domains cannot receive mail — use a real recipient address instead.

- **HTTP status**: `422`
- **Type**: `validation_error`
- **Name**: `RecipientDomainNotAllowed`

## What to do

The request body parsed, but one or more values are invalid. Read the `details` array: each entry names the failing field (`param`, a dotted path like `to[0].email`) with a human-readable reason. Fix the named fields and retry.

## Related

- [E04xxx — Email sending & delivery](/docs/api/errors/E04xxx): every code in this range
- [Errors](/docs/api/errors): the full wire contract, status mapping, and error catalog
- [Error handling](/docs/guides/errors): branching on `type` and `code`, validation details, and `vendor_code`