Email Responses from Helpdesk not being sent

We are using Helpdesk for HR support. When a user emails in everything happens as it should, except for a couple of users. They receive no emails from the system at all - no acknowledgement, no responses. I have run an Exchange mail trace for the inbound email address and I can see plenty of emails coming in, but none for those two users. It's almost as if they aren't being sent or are being blocked somewhere before they get to us.

Has anyone else seen this behaviour? Does anyone have any ideas?

Thanks!