Once you have been able to sign in to view the secure email, a reply button is displayed that you can click on the top left-hand side. This will display an email editor allowing you to compose your reply. You can add optional attachments by clicking the 'Choose File' or 'Browse...' button above the email body.
Once you have finished writing your message and added any attachments required, you are ready to send your message securely.
Please note: before pressing 'Send Secure' it is recommended that you ensure the 'Send me a copy of this email' option is ticked so you have a copy of the email.