What happens if a contact responds via email forwarding?

Hi there,
 
If your end users / contacts forward the ticket notifications to a third-party. A third-party is someone who isn't the requester or a CC, assignee, or follower on the ticket.
 
And if a third-party replies to a ticket notification from an email client. The reply becomes a private comment (internal note) on the original ticket. 
 
For more information, please see this article: Third party replies to ticket notifications
 
I hope this answers your question. Thank you!