Use ActionMailer config to send plain through plain Mail

plain_mail.rb

Mail.defaults do
  delivery_method ActionMailer::Base.delivery_method, ActionMailer::Base.smtp_settings
end

mail = Mail.deliver do
  to      'dmitry.polushkin@gmail.com'
  from    'Mikel Lindsaar <test@9flats.com>'
  subject 'First multipart email sent with Mail'

  text_part do
      body 'This is plain text'
  end

  html_part do
      content_type 'text/html; charset=UTF-8'
      body '<h1>This is HTML</h1>'
  end
end