It might be that an agent communicates with the client, then transfers the ticket to a new agent and writes some internal notes (which will never be visible to clients, and there will be no notification sent either), so that the next agent will not have to check all threads of the ticket.
@bansalvaish Also email created from a PHP application not open tickets.
This can be a security measure but how if an abuse report come to the abuse email address that is managed by the ticket system?
If this is a feature you should maybe let ticket open but warn with a banner that email comes from not a fully authenticated email address so can be a security issue reply or trust the ticket.
I use live helper chat on my website and tested that if someone send an offline message and this message is delivered to my support ticket email address the ticket is not opened, UVdesk mark the email as read and if I don't see this email in the email inbox I'm not informed in UVdesk about the support request email because no ticket are created.