All users are receiving emails on their Blackberry except for one user.
If I have read this correctly you cannot receive mail. That's different than the Send As issue, unless MS is really screwing with us and gave us a Receive As issue
If this the case, check the MAGT log for your email address/username. You will find some entries in there that should reference log on failures or something.
If you can receive but not send then follow Penguins advice and take yourself out of the admins group. That is the best solution to fix this.