Ok, I've done a couple of these setups for clients running SBS 2003 or Exchange 07.
There are a few settings which need to be enabled on the server for it to work, as well as a few open ports.
1. RPC over HTTP(S). I've read conflicting reports on whether or not this needs to be enabled. Our clients have mobile users anyway, so this has always been enabled in my case. Ask whoever runs your server.
2. Port 443 needs to be open. If you have OWA working in a browser over HTTPS, you're all good.
3. OWA needs an SSL certificate. It's fine if it's the default self signed one created by Exchange.
4. Exchange Activesync needs to be enabled.
So, from here you need to enter the following details into your phone:
1. Your full email address
2. The Server address, which is the address of the server as seen by the internet, not your local network. This is usually
https://webmail.domain.com. Leave out the /Exchange or /owa.
3. The domain. Check the logon screen of your work PC. Hit the Options button after Ctrl-Alt-Del and you'll see you have Username, Password and Domain. Enter the domain name into your phone exactly as you see it on this screen. Domain\Username has never worked for me. Same with using the email address as the username or using / instead. However, I have had success with leaving the domain out once.
4. Your password
5. And choose to use SSL (and accept the certificate if it prompts you).
On one iPhone I've had more success with entering the IP address of the Exchange server rather than the web address. It found the server fine, but wouldn't verify the details. Jump on a PC and ping the OWA address from an external PC to get the IP.
I've found it pretty "hit and miss". Some companies have no problems with iPhones, others do. I've even had one client get a replacement iPhone and that had trouble connecting to Exchange when the original phone connected within 30 seconds!