If you have the ability to manage the server, Exchange 2003 and greater has a way to specify ip addresses that are allowed to relay. I have also setup the IIS smpt server to send email using wwipstuff. I set the smpt server to allow sending email only from the computer's ip address.

>I'm getting error messages when trying to emails to recipients who are not on our network. Exchange is saying unable to relay for x, So, am I going to have to setup Blat or some SMTP server to do this?