Customer replies are not recording to ticket thread

[suzanneabate]

I've looked up a few older posts on this issue but none seem to adequately address the issue. Our SD is setup to require email addresses from customers when they submit requests. 

I can confirm that customers are receiving the notifications via email (both automated notifications and unique replies sent from our reps) but when they reply to those notifications from their email their replies are no longer populating to the ticket thread. 

[Quoc Nghia_ TRAN]

Could you investigate that any information on Audit Logs?