Right so to continue on, where BB as a product is different is that you don't configure any email accounts on the handset itself, you will be doing it through the web interface of My T-Mobile (once you buy the service add-on).
It'll go something like this:
1) log into My T-Mobile, and you may or may not need to switch your "preferred device" to a real BB to get the next options; you'll need to play with it. You *may* need to enter the PIN (does the BBConnect S60 software issue you a PIN??) like a real BB device; a PIN is sorta like an IMEI, but not really -- it's like.... like a serial number for your hardware, but it's used for other stuff (see below about Service Books). All real BB devices have a PIN, and that PIN is associated with T-Mobile's BIS service.
2) click on the "support for my device", and then on the right there will be a menu option called BlackBerry Email Accounts. You click that to get to a special page. There may be other ways to get here, but that's how I do it.
3) you first set an email address for your BB device (well in your case your E60), it will be @tmo.blackberry.net. Don't give this out to anyone! (spam spam green eggs and spam) I repeat, do NOT give this out to anyone, and choose something more obscure than common. (I chose a sorta common one, and started getting spam right away -- I changed it to something obscure and now it's clean)
4) now use the webUI wizard in front of you and add email accounts -- I've done GMail and Yahoo and so on without any problems. It's as simple as entering your email address and password, and the T-Mobile webUI figures out the rest.
5) on the left, you see this menu option called "Service Books" -- you click that, and then click the option that says "Send service books to my device". Think of this as sending a download configuration.
That's pretty much how it works -- your question about if your company ran a server (that would be called BES, BlackBerry Enterprise Server) would be done by IT; they would have the PIN of your BB device (or S60 BBConnect) in the server, and configure it for you and push Service Books out to you, just like BIS with T-Mobile.