carrierwave

Mail.defaults

Sets the default delivery method and retriever method for all new Mail objects.
The delivery_method and retriever_method default to :smtp and :pop3, with defaults
set.

So sending a new email, if you have an SMTP server running on localhost is
as easy as:

  Mail.deliver do
    to      '[email protected]'
    from    '[email protected]'
    subject 'hi there!'
    body    'this is a body'
  end

If you do not specify anything, you will get the following equivalent code set in
every new mail object:

  Mail.defaults do
    delivery_method :smtp, { :address              => "localhost",
                             :port                 => 25,
                             :domain               => 'localhost.localdomain',
                             :user_name            => nil,
                             :password             => nil,
                             :authentication       => nil,
                             :enable_starttls_auto => true  }

    retriever_method :pop3, { :address             => "localhost",
                              :port                => 995,
                              :user_name           => nil,
                              :password            => nil,
                              :enable_ssl          => true }
  end

  Mail.delivery_method.new  #=> Mail::SMTP instance
  Mail.retriever_method.new #=> Mail::POP3 instance

Each mail object inherits the default set in Mail.delivery_method, however, on
a per email basis, you can override the method:

  mail.delivery_method :smtp

Or you can override the method and pass in settings:

  mail.delivery_method :smtp, :address => 'some.host'

Source | Google | Stack overflow