you can't set it per sub site or site collection (out of the box). what ever it is set at for the web application or the global is what it's going to send as.
if you only have one web app, then all of your sites are under it. change the outgoing email settings for that app and it will change it for the site. again, all your sites will use this setting. you can't modify it for one subsite and use something else for another subsite.
understand?
