When you're sending a reply to a helpdesk-ticket from within your email software, you can send instructions to the help desk app by including specific text in your email subject.
The updated list of all the commands can be found in the in-app help, just click the bottom "help" link on the "admin - email settings" page.
Remember that you need to preserve the ticket-number in the subject line at all times. Otherwise the helpdesk app won't know which ticket your email refers to - and will create a new ticket.
Keep in mind that you can simply forward an email to the helpdesk app. For instance, if someone sent it to your personal mailbox by mistake. Simply forward this email to the helpdesk mailbox and it will "extract" the original sender from it and create a new support ticket on his behalf.
Make sure that: