Setting the Masquerade Domain (IIS 6.0)
The masquerade domain replaces any local domain name used in any Mail From lines in the protocol. The replacement occurs on the first hop only.
You must be a member of the Administrators group on the local computer to perform the following procedure or procedures. As a security best practice, log on to your computer by using an account that is not in the Administrators group, and then use the runas command to run IIS Manager as an administrator. At a command prompt, type runas /User:Administrative_AccountName "mmc %systemroot%\system32\inetsrv\iis.msc".
To set the masquerade domain
All replies to such messages will be routed through the SMTP virtual server that uses the masquerade domain.