what error is displayed on the blackberry when trying to activate?
is GPRS or EDGE in capital letters on the blackberry phone?
try to follow the activation email that's sent from the blackberry.
(leave email server address empty)
it's possibly stopped by a spamfilter (corporate or on user's inbox)
it could contain the wrong password (not likely if you're doing it yourself)
it could be that the BESAdmin service account doesn't have the right to access and administer the user's email box. (check for receive as, send as and administer information store right for the BESAdmin on the mailbox store in exchange system manager)
check if you open Exchange System Manager as the BESAdmin and open up the mailbox store to see the mailboxes if you can see that particular user's mailbox.