To configure the iPhone 3G for email use:
Tap Settings,
Scroll down to and Tap Mail, Contacts, Calendars
Tap Add Account
Tap Microsoft Exchange
Enter your company email address
Enter your companie's email domain (i.e. Mail.Yourcompany.com)
Enter your Windows Log on Username and password.
and press next.
if it asks you to confirm a security certificate, allow it to, and follow the wizard through to the end.
Its literally as simple as that.
No server/firewall configuration needed.
Hope This Helps!
kevinredfern