Dry-run validation + pricing for an order

Letter body. Supports {{first_name}}/{{last_name}}/{{greeting}} merge fields.

Send with no return address at all. Only honoured when no return_address is given and the account has no default; undeliverable mail is then destroyed. Without it, such an order is a 400 no_return_address_unacknowledged.

Client-supplied key, 1-255 characters. A repeat with the same key and the same body returns the original order (200, idempotent_replay: true) instead of creating a second one. The same key with a different body is a 422 idempotency_key_reused; a repeat while the first request is still running is a 409 idempotency_in_progress. Also accepted as the Idempotency-Key header; the body wins.