Shap, these guys may be talking over your head a bit.
If you work for a company that has its own Microsoft Exchange Server and multiple Blackberry users then it might be cost-effective to purchase your own BES (A server software package that runs on your Exchange server or another machine). A standalone BES is not very affordable if you are a small business or a lone person.
An alternative is to use a service such as
www.mailstreet.com. What they do is run Exchange Servers that you can access over the Internet. You'd purchase an Exchange email account for yourself and your assistant ($10/month each) and then add the BES option to your account (another $10/month) then your Blackberry will work like you want. If you are using T-Mobile then they will also charge you another $10/month for their BES-capable service on your phone.
Those are basically your two options depending upon the size of your company and how many handhelds you have to support.