Bashar, by "outside" I take it that you mean, "I want it to come from domain.com" (your domain).
To do this, you'll have to change the MX (mail exchange) record in the domain registration to point to the IP address of your SMTP server. Of course, this will mean that any existing email server is no longer going to be processing email for this domain; everything will be routed to the IIS machine. If this isn't suitable then you will not be able to truly send email that originates at your domain.
One workaround is to use your domain in the "Reply-to" and "FromAddr" of your email. Indeed, seeing your domain in the Reply-to field is enough to convince most people that the email did actually originate from there. However, if the FromAddr points to an actual email server on one IP address and the mail actually originates from a different IP address (e.g. your IIS server) then many email relay servers will block it as likely spam.
Also, I recommend that you look into AspNetEmail for managing email (really SMTP) programming. In addition to having a superb control, they also have a very rich body of technical support articles and a first-rate support program. These people really know SMTP. You could read their content even without buying anything but a lot of the information is cast in terms that are specific to their control. Of course, I also think it is a good idea to support vendors that help you.