Case Updates in threaded view

Hello,

When I create a case and relate it to a contact the system correctly send an email to the customer.
I expect that if the customer reply to the email, his message will appear on the “Case Updates Threaded” field.
No update appears instead.
I don’t have auto-import nor auto-case creation for the sending account because I prefer to import it manually.

Do you have any suggestion?

Thanks,
Stenio