HI Subbarao,
Usually, the issue this kind of scenario is that Maileater cannot find the unique string in the mail to identify the existing ticket for an update or the incoming mail did not match an update mailbox rule, thus it creates a new ticket instead.
If the KB article mentioned by Chi_Chen does not help, then we need to interrogate the detail of mail that was sent and mailbox rule that was supposed to process it in order to effect the update.
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Kind Regards,
Brian