def part(params = {})
      new_part = Part.new(params)
      yield new_part if block_given?
      add_part(new_part)
    end
def part(params = {})
      new_part = Part.new(params)
      yield new_part if block_given?
      add_part(new_part)
    end
Allows you to add a part in block form to an existing mail message object

 Example:

  mail = Mail.new do
    part :content_type => "multipart/alternative", :content_disposition => "inline" do |p|
      p.part :content_type => "text/plain", :body => "test text\nline #2"
      p.part :content_type => "text/html", :body => "<b>test</b> HTML<br/>\nline #2"
    end
  end