I'm having an issue with one Exchange 2003 user. The user has a 38 GB mailbox, so I was testing some things to bring the user into a new workstation since there were problems using cached mode. One of the things I tried was putting all his mailbox defaults to 0, since I figured this would make the mailbox size unlimited. Then the user stopped receiving emails. Anyone who sends him an email gets an NDR saying the mailbox is full and Exchange won't continue sending it. So, then I found that the unlimited mailbox size is simply unchecking all the boxes in the defaults tab. After I unchecked all the boxes, the same NDR reply comes up whenever sending to this person.