mailer1.rb

class UserMailer < ActionMailer::Base
  # ...

  def daily_news(user)
    mail(to: user.email, subject: "Daily newsletter")
  end

  private

  def accepts_delivery?(address)
    user = User.find_by_email(address)
    user.present? and user.send("wants_#{action_name}_email?")
  end

  def mail(headers = {}, &block)
    super(headers, &block)
    message.perform_deliveries = message.to.all? do |address|
      accepts_delivery?(address)
    end
  end
end

mailer2.rb

class User < ActiveRecord::Base
  # ...

  def wants_daily_news_email?
    email_confirmed? and # ...
  end
end

mailer3.rb

# deliver conditionally
UserMailer.daily_news(user).deliver

# force delivery
UserMailer.daily_news(user).deliver!