You do not need IMAP for IPhone 3G, Iphone 3G support Exchange 2003 /2007 natively using Microsoft Active Sync. ( you only setup imap4 for older Iphone),
What is the user name you use: You should use either your email address such johndoe@abc.com, or use your network user name plus your domain name.
Check with your SSL port setting and make sure your firewall is configured correctly.