Create a ruby file in #{RAILS_ROOT}/config/initializers/ with the following code

# Load mail configuration if not in test environment
if RAILS_ENV != 'test'
  email_settings = YAML::load(File.open("#{RAILS_ROOT}/config/email.yml"))
  ActionMailer::Base.smtp_settings = email_settings[RAILS_ENV] unless email_settings[RAILS_ENV].nil?
end

And then create #{RAILS_ROOT}/config/email.yml with the following

development:
  :address: smtp.server.com
  :port: 25
  :authentication: plain
  :user_name: user
  :password: pass
production:
  :address: smtp.server.com
  :port: 25
  :authentication: plain
  :user_name: user
  :password: pass

And that's all.