Currently, there is no link between a work item created from an email sent to a closed work item and the closed item. This means that when a user sends in another email to the closed work item, instead of attaching to an existing work item that was created from an email sent to a closed work, it will create a brand new work item.
Example:
Email 1 creates Ticket 1.
Ticket 1 gets closed.
User uses email 1 to send email 2 to the same address.
Because Ticket 1 is closed, email 2 will create Ticket 2.
User uses email 2 to send email 3 to the same address.
Instead of attaching to Ticket 2, email 3 will create Ticket 3.