Ruby on Rails

I am studying Peter Cooper's book Beginning Ruby.
According to the author ActionMailer can be used independently of Rails.
So I installed the ActionMailer gem in a dedicated gemset and wrote the code below which is an extract from the book:

require 'action_mailer'    ActionMailer::Base.smtp_settings = {      :address => "smtp.mail.yahoo.com",      :port => 465,      :authentication => :login,      :user_name => "username@yahoo.com",      :password => "password",      :openssl_verify_mode => :ssl  }    class Emailer < ActionMailer::Base    def test_email(email_address, email_body)      mail(to: email_address, from: 'username@yahoo.com', subject: 'action mailer test', body: email_body)    end  end    Emailer.test_email('username@gmail.com', 'This is a test e-mail!').deliver_now


When I run the rb file containing the above code I receive a long traceback (21 lines) with the following error message:

/home/krfg/.rvm/rubies/ruby-2.5.1/lib/ruby/2.5.0/net/protocol.rb:189:in `rbuf_fill': end of file reached (EOFError)


Why do I receive this error?
Also I tried run test_email on an instance of the class Emailer but I receive undefined method `deliver_now' for #<Mail::Message:0x0000000002256b68>