send subdomain for the Return-Path address.
You can provide a custom Return-Path address when you add a new domain in the Dashboard under Advanced options, or by setting the optional custom_return_path parameter when creating or updating a domain via the API or with a domains CLI command.

- Must be 63 characters or less.
- Must start with a letter, and end with a letter or number.
- Must contain only letters, numbers, and hyphens.
testing), as they may be exposed to recipients in some email clients.