Address in field From address should generally be the same as user name in your outgoing mail settings. In most cases, you will not be able to use an email like "no-reply@example.com" and at the same time have "helpdesk@example.com" as a user name in your outgoing mail settings. Most mail servers (like Gmail, Yahoo, Yandex, etc.) don't allow fake addresses in FROM field to avoid spam attacks. If you have your own server and your system administrator allowed fake addresses you may use different addresses in FROM field.
Note: Regardless if you can change the email or not, you can still edit the identity that the recipient sees. For example, if you set From address as {%response.author%} from MyCompany <helpdesk@example.com> and Greg answers a ticket, the customer will receive a message from "Greg from MyCompany".