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 => "11.22.33.253",   :domain => "mail.example.com",   :port => 25,   :authentication => :plain,   :user_name => "my_username",   :password => "my_password" }

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

John