I'm getting this error when trying to authenticate with Exchange Server from Ruby on Rails:

504 5.7.4 Unrecognized authentication type

config.action_mailer.raise_delivery_errors = true
config.action_mailer.perform_deliveries = true
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  :address              => "x.x.x.x",
  :port                 => 25,
  :user_name            => "xxdomain\xxuser",
  :password             => "xxxxxx",
  :authentication       => :login,
  :enable_starttls_auto => true
}

I've tried all sorts of combinations of configuration settings, including changing the settings to use "plain" authentication, adding the domain, setting enable_starttls_auto to true, false, and removing it entirely, removing the port. Nothing has worked.

Any ideas?