Update request using email command?

Update request using email command?

Hi,
 
I did try to update ticket by using email command but it didn't work. I used @@ in the email subject then type @@Category=General@@, @@RequestID=xxx@@ something like this. Everytime i did it always create new request with new request id.
 
It' working well with create new ticket with using email command but not for update. So please could you help or advice the way to update ticket with email command.
 
Thank you.
Sakarin N.

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