Is it true that the space-time postman sends text messages?

You can't. The space-time postman is a fictional concept, and there is no way to send it, because the space-time letter is only an imagination in science fiction works, and there is no real existence. So the space-time postman is fictional. Postman, formerly known as postman, is responsible for carrying big bags and small bags, collecting mail, sorting letters and distributing them.