Exchange 2003

I have a couple of users that have not recieved internal emails. One example of this is a user say name@domain.com sends an email to 4 other users in the same domain i.e. TO: user1@domain.com, user2@domain.com……user6@domain.com. In this example user2@domain.com did not get the email. I checked the message tracking on the exchange server and it tells me it was delivered. One other strange thing is that user3@domain.com replied to all and user2@domain.com recieved the reply, but not the orginal. This user is a bit of a BS-er, but now that it has happened to 3 other users, I now believe there is an issue. Can someone please help me? Thanks you in advance for the help.