(1) When sending from Lotus Notes, the client's mailer lookup the current Location document for the "home server" to post the sent message. The Domino Router that will do the delivering is the one located on this "home server".
(2) When sending from BlackBerry device for the user local to the BES, the sent message is posted to the BES' mail.box. The Domino Router that will do the delivering is the one located on the BES.
Find out the DIFFERENCE(S) between the 2 Domino Routers and you shall be able to resolve your issue. This is more of a Domino mail routing issue rather than a BES one.
Native but 4th class citizen of a nation governed by idiots who import congestions & contention.