send_mail_user.rb

# Controller 

SendEmailToUser.perform_async user.id

# ...

class SendEmailToUser
  include Sidekiq::Worker
  sidekiq_options :queue => :default, :retry => true, :backtrace => true

  def perform user_id
    begin
      user = User.find user_id
    rescue ActiveRecord::DocumentNotFound => e
      logger.warn "User não existe, ignorando"
      return
    end

    Mailer.send_mail_user(user).deliver
  end
end