It sounds like what's happening here is that the spam messages are being picked up by the BlackBerry Enterprise Server (BES) for redirection before the spam filtering software for your Lotus Notes mailbox deletes them.
If many BlackBerry users in your enterprise have this issue, there is a setting that may be made on the BlackBerry Enterprise Server that will cause the BES to briefly delay redirection so your Lotus Notes Domino spam filters have time to work before the messages are redirected. You may want to ask your BES administrator about this.
To remedy this for yourself, the BlackBerry has filters that you may use to configure which Lotus Notes messages get redirected and which do not get redirected to your mobile device. These filters may be configured with wild cards, multiple to and from addresses, etc.
If you open the message list (inbox) on your BlackBerry smartphone, and choose "Help" from the menu list, there is an "Email" category where you'll find a discussion of how email filters work, as well as how to configure them.
I've also witnessed some spam coming directly from cell providers, and it is actually directed at a device via PIN or SMS. This way, it never hits the BES.
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