Hang on.....
You've got Microsoft Exchange
You've got a Blackberry
Why OWA ???
If you don't want to spent the $$$ on BES, try BPS.
Probably because RIM are incredibly late on delivering SBS2008 versions of BES/BPS? and haven't even considered Exchange2010?
BIS should be sufficient, although BlackBerries are my LEAST favourite smartphone. Every other device does ActiveSync in a relatively proper way (save for Apple*), yet RIM persevere with reinventing the wheel and charging for it.
(*
iPhone and iPod touch: Troubleshooting Exchange ActiveSync 'Push' issues - read "solution" 2
)
Incidentally, to the OP, I have several BB BIS users and know the config isn't always "obvious".
I don't have a crib-sheet of a proper BIS config to work from, but items to check are:
1) If you're not prompted for a separate 'domain' in the config, are you then prefixing your username thus: domain\username
2) Are you using the "https://" prefix and "/exchange" or "/owa" suffix to the server address? or are you just quoting the server address?
There are a few more, and a certain amount of it depends upon whether your OWA is actually set up properly as well. It would also help to know whether you're using Exchange 2003, 2007 or 2010 for OWA?
Cheers,
Mike.