I think I have a solution for my 2 year long problem. I have a blackberry on BIS and my company is small. We use a microsoft webmail type program and outlook with pop 3 in the office. We forward the emails directly from webmail to our .blackberry.net addresses. The problem is, the server has no spam blocker, and even if I forwarded from outlook their spam blocker is weak and id have to wait 15 mins to get emails, which I could never do.
I changed the "forward to" on the webmail from my @blackberry.net account to my .mac account (use gmail or any other webmail with spam blocker) then i forwarded the .mac emails to my blackberry.net account. It sounds complex but it takes about 30 seconds to do. The .mac account is able to filter the emails before they come to my handheld and no settings changes are needed on the device!
So far ive gotten no junk to my blackberry and the emails come in just as fast as they always have.
Good luck, and if you have any questions, just ask!