It’s common practice to just let Rails ignore the mail errors. In your config/environments/development.rb file add, uncomment or modify:
# Don't care if the mailer can't send
config.action_mailer.raise_delivery_errors = false
You can also set this:
config.action_mailer.perform_deliveries = false
See the documentation here http://edgeguides.rubyonrails.org/action_mailer_basics.html#action-mailer-configuration
You can also set the delivery method to :test, but I have not actually tried that
config.action_mailer.delivery_method = :test