Have a look at the BlackBerryOutbox queue and see if you have users with high pending messages....you can also see this from the domino console as well.
I'll assume you have added the BES to the "LocalDomainServers" group and all that other "good stuff" they describe in the install guide?
Try running a trace on the mail server(s) from the BES domino console.
Example....trace server1/OU/O.....also do a "sh us debug"....without the quotes. This will show if there are any TCP connections to the BES server from user(s) wrkstns.