If you have new tickets that can be created when sending emails, you can use subject-specific replies to update tickets from emails too.
For example, you could create a new ticket by submitting an email with the subject 'Access Intranet' to firstname.lastname@example.org, this will create a new ticket with that specific subject.
If you then wanted to update the ticket, you could either reply to the ticket confirmation email, or create another email that references the subject-specific features. By default, this would be "Ticket 1 Status --> Access Intranet".
Additionally there are ways of updating parts of tickets via an email, such as updating the status of the ticket by using: Status:YourStatusHere
I believe the most reliable way to update a ticket with a reply whilst sending it directly to the client is by replying to the initial ticket email that was submitted/created which has the pre-filled subject-line ready for you.
You can forward that email conversation to your WHD email address and change the subject to:
Ticket:999 Action:TechUpdate MinutesWorked:11
The Action: TechUpdate will add the email thread to the ticket.
MinutesWorked is optional.