Make sure this is set in /etc/postfix/main.cf:
smtp_generic_maps = hash:/etc/postfix/generic
When setting a generic alias, don't use "http" like the link you provided - you would use www-data with Ubuntu.
echo 'www-data yourusername@yourdomain.com' >> /etc/postfix/generic
postmap /etc/postfix/generic
service postfix restart
A side note: Sendgrid is awesome but since Rackspace bought MailGun the MailGun service is pretty much free for Rackspace customers. Hope this helps!