Is it possible to log into a user account on OWA from a computer already logged into to the domain with a different user account?
My example,
There is a Point of Sale system logged into the sales account, a user needs to be able to access OWA to view their email while working the sales floor. When the go to the OWA page it automatically logs them in as the Sales User, and logging out does not work. Is this a server setting that needs to be changed?
Yes it is. You can have a look at this KB Article for how to do it with forms based authentication. I believe you can also use basic authentication on the OWA site via IIS in order for the browser to prompt you for authentication. If you go that route I'd highly recommend also using SSL for your OWA install.