Devise sends registration confirmations, forgotten password messages, and other email messages. These will originate from the main domain by default, but you can modify the application so these emails originate from subdomains.

One of the approaches to making that work is to give the url_for helper a :subdomain option.

# app/helpers/subdomain_helper.rb

module SubdomainHelper
  def with_subdomain(subdomain)
    subdomain = (subdomain || "")
    subdomain += "." unless subdomain.empty?
    host = Rails.application.config.action_mailer.default_url_options[:host]
    [subdomain, host].join
  end
  
  def url_for(options = nil)
    if options.kind_of?(Hash) && options.has_key?(:subdomain)
      options[:host] = with_subdomain(options.delete(:subdomain))
    end
    super
  end
end

The next step is crucial. Make sure that Devise mixes in your new subdomain helper in config/initializers/devise.rb

  Devise.setup do |config|
    config.mailer.class_eval do 
      helper :subdomain 
    end
  end

Now when you do a link_to in your Devise mailer template, you can specify a :subdomain option.

link_to 'Click here to finish setting up your account on RightBonus',
  confirmation_url(@resource, :confirmation_token => @resource.confirmation_token, :subdomain => @resource.subdomain)

**for devise-2.2.3
This seems to be sufficient :

link_to 'Click here to finish setting up your account on RightBonus',
  confirmation_url(@resource, :confirmation_token => @resource.confirmation_token, :subdomain => "my_subdomain")

NOTE You must configure the default_url_options[:host] value in your environment configs for this solution to work properly.

# config/environments/development.rb
config.action_mailer.default_url_options = { :host => 'localhost' }

Configure each environment accordingly and things should work as expected.

If this is not working for you and the subdomain doesn’t get added to the url consider the following:

When using edit_password_url
The url_for helper receives a String with a url as options instead of a Hash, so the :subdomain never gets added.

link_to 'here', edit_password_url(@user, reset_password_token: @user.token, subdomain: @user.subdomain )

Using edit_user_password_url (replace user with your own resource name).
The helper method gets called twice. First with the options Hash, adding the :subdomain. Then with a String now containing the right url with subdomain.

link_to 'here', edit_user_password_url(reset_password_token: @user.token, subdomain: @user.subdomain )