These errors aren't very Wagn-specific (they come from Rails and/or Postfix and have to do with what your configurations allow you to do), but there are a couple of wagn-specific solutions:

On wikirate, for example, we have these in config/application.rb:

config.email_defaults = { :from => 'noreply@wikirate.org' }

config.action_mailer.smtp_settings = { :address => 'kasper.wikirate.org' }

The first sets the default from address for all outgoing email. In my experience, if the domain name (eg wikirate.org) matches the hostname of the current server (as understood by postfix) or the address directly configured in smtp_settings (see above) then postfix is happy.