Please implement Variable envelope return path (VERP)
VERP is a standard industry practice.
https://en.wikipedia.org/wiki/Variable_envelope_return_path
It has been around for a long time and many places use it. Including your competitors.
With VERP, you can use the sender email address to incorporate the ticket ID association so that the end user (customer) doesn't have to worry about the subject line. Also, it allows me (the CSR/etc) to reply directly to the email, and then my reply would go to Zoho, get logged, and then my reply routed to the intended party or parties. This is a much more effective structural path for support ticket email handling, and quite honestly, I'm surprised you guys don't use it.
The issue that I run into on occasion is that people using certain email programs, the gmail app on iPhone in particular, the [##534##] ticket ID in the subject line is wiped out by the app, so every time they reply, a new ticket is opened. This is super annoying for me, and super annoying for them as well. I troubleshot this with one customer after he was the 3rd person to have this occur, he replied to one of my email from 4 different methods and the gmail app was the culprit.
You can't predict how an email client (system) is going to react to a tag in the subject line, and you certainly cannot anticipate when a customer is going to replace the subject line, thinking that is no big deal. It's the little things like this that make things just a bit annoying, for me and my customers. I shouldn't have to train my customers to leave the subject line alone. Incorporating the ticket ID into a VERP within the email address is much more robust.