Why does postfix not let me send directly to a user, but the alias works?

I have these entries in my /etc/aliases file on my main site, example.com:

scott:             admin@otherdomain.com
payment_support:   scott

If I email payment_support@example.com, it is correctly delivered to admin@otherdomain.com

If I email scott@example.com, it bounces back to me with 500 5.1.1 scott@example.com Address Error

What the-- ?