User unable to get emails when receipents use the reply function

I have a user that doesn’t get replies to her emails when the person she emails uses the “reply” function. If they start a new email to her, she is able to get it, but if they hit “reply”, she doesn’t. No error on their end or our end. Have looked at Mesg tracking, but if she didn’t get it, nothing to find about that mesg. Any ideas?

We use exchange 2003 on Server 2003 R2

Thanks!