Fixing 'Relay access denied' in Postfix
So and where did you specify for which domains Postfix should be responsible? Nowhere. This is why you get this error. Add your domain to the line of mydestination. And then you need at least a local user named developer or put an alias into /etc/aliases.
But you have to read this documentation as an absolute minimum before you start with Postfix: http://www.postfix.org/BASIC_CONFIGURATION_README.html