Recently on our exch 2003 server we had a sales distribution group setup that stopped working. After looking into it I determined that it was only sending to 1 of the 5 people in the list.
I have now just setup another distribution group which uses the same 5 people and again the same person is the only one that receives it.
When looking in mail tracking obviously the user that receives it shows. However there is no record in the mail tracking for the other 4 users.
I have updated the recipient service as suggested in another question here.
I have tried a server restart.
I have checked spam filters and rules on the clients side.
This problem happens with both internal and external mail.