abeermuh
You will need to look at that email's headers to see if it is In-Reply-To or References any other email in that Ticket thread. If so, that's the reason. We mainly match on the In-Reply-To and References headers. If that doesn't match we attempt to decode the Message-ID and try to pull info from that. Lastly, if that fails we try to see if there is a Ticket Number in the subject (eg. #12345
). If none of those match then it will open a new Ticket.
With this being said, we do not use the full subject; we only try to extract a Ticket Number as a last resort. So if the subject is simply "ABC" then this is not the cause of the issue; rather it's most likely the email headers.
Cheers.