You can do this by adding settings.py such as :
EMAIL_BACKEND = 'django.core.mail.backends.smtp.EmailBackend'
EMAIL_HOST = 'localhost'
EMAIL_PORT = 25
EMAIL_HOST_USER = ''
EMAIL_HOST_PASSWORD = ''
EMAIL_USE_TLS = False
DEFAULT_FROM_EMAIL = 'Whatever <email@example.com>'
Also make sure that a fully qualified domain name is set up on your server.
Then you need to have these lines in your /etc/postfix/main.cf:
myhostname = mybox.example.com
mydestination = localhost.server.com, localhost, example.com