I'm trying to relay messages from different websites on my Windows server.
The SMTP authentication in IIS6 allows for one username and password.
I have purchased two smtp relays through your company but it does not allow me to change the username. Can you help with how to change the username on your system or how to set up multiple authentications in IIS smtp server?

Or is there a way to have an smtp relay set-up with my server IP? This way I could have multiple websites access this.

If you have multiple domains signed up for SMTP relay in your user account, any one of the username/password will work for all domains.