We have just recently imported all our old tickets from Zendesk and I've noticed that if a client responds to a imported ticket via email using the email sent from the old system it creates a new ticket and doesn't reply to the imported ticket.
Is there anyway to prevent this with out having to merge the newly created ticket and rather have the response added to the existing ticket?
You'll need to ensure the subject matches up with the way that tickets are tracked in FD.
So - in Freshdesk they track tickets in the subject with the call ID e.g. [#123456] if this is not in the subject the emails will create a new ticket.
Not sure how you can get around it but I thought I'd explain why.