Seeing this is something I get asked to provide often it was a nice little surprise to see this in the latest builds of OS 5.0.
What I have found out via a chat with T-Support ..
This is something carriers ca include in the latest OS 5 builds. Right now it seems only Sprint and Verizon offer it. I need to check a at&t Curve 2 but it's not on the Bold 2.
Still havn't confirmed if BES 5 is needed but I don't believe so.
Nice way to sort your email in their own "inboxes" without having to run the script on BES.
CIO.com had a nice overview of it
BlackBerry Tip: How to Separate BIS and BES E-Mail in OS 5.0 - CIO.com - Business Technology Leadership