Symptom: POST to the emails endpoint returns 422 instead of an email id.
1. Confirm the cause: read the error message in the response body. Resend's error schema puts the specifics in the message, including which field failed and why. Do not guess; the message lists the missing fields.
2. missing_required_field: the request body is missing one or more required fields. The message names them; add each one.
3. invalid_attachment: "Attachment must have either a content or path." Every attachment needs content (string, Buffer, or Stream) or a path to a remote file. Attachments with only a filename, or with neither, get this error. For larger files prefer path to a remote resource.
4. invalid_parameter / missing_required_parameter: a parameter must be a valid UUID, or a required parameter is absent. Check the value format and that the parameter is present.
5. Note the nearby 400: validation_error ("An error was found with one or more fields") is the same family with more detail in the message.
6. Verify: re-send and confirm a 200 with an email id. If you build payloads programmatically, log the exact body you sent so the next 422 is a one-line diagnosis.