ActionView::Template::Error: Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true

My ruby on rails action mailer runs all good in development environment, but in production environment, it keeps throw:

ActionView::Template::Error: Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true

My development config is

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  :port           => xxx,
  :address        => 'smtp.example.org',
  :user_name      => 'xxx@example.com',
  :password       => 'xxxxxxxx',
  :domain         => 'xxxxxx.com',
  :authentication => :plain,
}

config.action_mailer.default_url_options = { :host => 'localhost:3000' }

My production config is

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  :port           => 587,
  :address        => 'smtp.example.org',
  :user_name      => 'xxx@example.com',
  :password       => 'xxxxxx',
  :domain         => 'example.com',
  :authentication => :plain,
}
config.action_mailer.default_url_options = { :host => 'example.com' }

My other environments are:

ruby 2.1.1
rails 4.0.3
sidekiq 
devise
devise-async

I have tried:

1. Add the following in a initializer file

   ActionMailer::Base.default_url_options[:host] = "example.com"
   

2. This answer here

Neither of them works.