Read this order
Letter → Rider: A document or written message being delivered or passed on. The emphasis is on the movement of information.
What changes in reverse?
Rider → Letter: News arriving in writing: a message, email, notice or document that brings a new development.
Rider → Letter ↗A practical example
Consider an application awaiting a reply. Rider + Letter can fit news arriving as an email. Letter + Rider begins with the email and emphasizes its delivery or forwarding. Check the actual message rather than inferring its contents from the cards.
Check it against your question
Keep the actual order and ask which part of the example fits your situation. An example illustrates a possible reading; it does not establish facts or promise an outcome.
The meanings behind the pair
Letter: Letter is a written message, document, notice or record. It draws attention to the information itself and to how it has been communicated.
Rider: The Rider brings news, a visitor or a new arrival. It usually adds movement and a relatively quick pace.
These examples show possible ways to read the pair. Your question and the other cards may change the meaning.