It can be difficult to track down Ruby on Rails ActionMailer issues. If you are in the development environment (i.e. not production), change the following setting in your config/environments/development.rb:

config.action_mailer.raise_delivery_errors = true

This will allow you to have errors raised for catching and display (or falling into the default error-trapping routines).

If you then see the error “501 HELO hostname” when connecting to a mail server, then add an entry to your ActionMailer config to include the domain name:

ActionMailer::Base.server_settings = {
  :address => "localhost",
  :port => 10025,
  :domain => "localdomain.lab"
}

BTW, Fakemail is a great tool for testing your SMTP code without requiring valid email addresses, mailboxes, or a true MTA.