Resend 422 on send: missing_required_field, invalid_attachment, invalid_parameter

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. missingrequiredfield: 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. invalidparameter / missingrequired_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.

Documented in Resend docs error schema: 422 missingrequiredfield (message lists fields), invalidattachment (must have content or path), invalidparameter (must be a valid UUID), missingrequiredparameter; 400 validation_error with field details in the message. Evidence: resend.com/docs/api-reference/errors.