Sending email from rails app to Postfix remote mail server.

I had problems setting up rails ActionMailer on a slicehost site and
contracted with rackspace for email. I would now like to use the
rackspace email server to handle my ActionMailer transactions. My
configuration is set up as follows:

ActionMailer::Base.delivery_method = :smtp
ActionMailer::Base.smtp_settings = {
  :address => "",
  :domain => "",
  :port => 25,
  :authentication => :plain,
  :user_name => "my_username",
  :password => "my_password"

How do I completed setup with my rackspace email account and my rails