Hi,
within Rails 2.3.3 a bug within the ActionMailer was introduced. You can see the ticket over here Ticket #2340. It's resolved in 2-3-stable and master so it will be fixed in 3.x and 2.3.6. 
For fixing the problem within 2.3.* you can use the code provided within the ticket comments: 
module ActionMailer
  class Base
    def perform_delivery_smtp(mail)
      destinations = mail.destinations
      mail.ready_to_send
      sender = (mail['return-path'] && mail['return-path'].spec) || Array(mail.from).first
      smtp = Net::SMTP.new(smtp_settings[:address], smtp_settings[:port])
      smtp.enable_starttls_auto if smtp_settings[:enable_starttls_auto] && smtp.respond_to?(:enable_starttls_auto)
      smtp.start(smtp_settings[:domain], smtp_settings[:user_name], smtp_settings[:password],
                 smtp_settings[:authentication]) do |smtp|
        smtp.sendmail(mail.encoded, sender, destinations)
      end
    end
  end
end