ZENDESK doesn't recognize previous mail when a customer reply on forwards

I'm using Zendesk SUPPORT 

The customer has to "reply" to the email sent by the agent to recognize the last comment (email) and it show mail cleary both zendesk and requster mail 

But If requster reply by forward , it will recognize the previous email as a last comment, and you will see the previous comment (email) as shown in the picture below, which is very inconvenient. 

 I'm sure all of you agents are facing similar issues like this, and I'm curious how you all solved it. 

 #Please type your reply above line ##

 I'm afraid that the customer will see the above warning from the ZENDESK system and assume that they only need to write above the line. 

<If the customer sends mail via forwarding>.

<Zendesk doesn't recognize the previous email, as shown below.>

Even though requster write a comment (email) above.  #Please type your reply above line ##
It recognizes the previous comment as a new comment, that is showen too messy both zendesk agent and requster  
Also, the customer will see the same comment as a new one, which may cause inconvenience to you. 

1. i was wondering how to guide requster to keep new comments when ask requster to reply to an email. 
2. also, may I know the detailed principle of recognizing new comments?
    I need to know the principle so I can find a way to solve it.