agenda-libre-ruby/app/mailers/note_mailer.rb

class NoteMailer < ActionMailer::Base
  def create(note)
    @note = note

    mail 'In-Reply-To' => "<event-#{note.event.id}@#{ActionMailer::Base.default_url_options[:host]}>",
      to: note.event.contact,
      subject: t('note_mailer.create.subject', subject: note.event.title)
  end
end