IT360 / ServiceDesk Plus is not receiving replies to tickets if user email is outside our domain

IT360 / ServiceDesk Plus is not receiving replies to tickets if user email is outside our domain

We can send emails to outside users from the tickets.
The ServiceDesk Plus mailbox is receiving the replies from outside users but when it fetches, the email disappears and does not get into the ticket. I don't see any filters preventing this.
We are using ME IT360 Professional 10.5.0 Build 10500.
We do want to receive the ticket replies, is there any reason why this is happening?