You can reply directly from your email client. Just click on the "reply" button, the reply will be routed via SimpleLogin and SimpleLogin will make it coming from your alias. Your personal email will stay invisible to the original sender.
Technically, the From header in your email is replaced by a special alias dynamically generated for each sender. When you reply, your reply is actually sent to this special alias and SimpleLogin will relay the reply back to the sender, making sure the email is sent from your alias. All information about your personal email address is removed during that process.