For Windows Server 2003, go to IIS 6.0 > Default SMTP Virtual Server > Right click Properties > Delivery tab > click Advanced.

Maximum hop count – use default value of 15
Masquerade domain – choose your public DNS name of your mail server (i.e. mail.yourcompany.com)
Fully-qualified domain name – use private IP of your mail server
Smart host – use private IP of your mail server

Hope this helps.