campaign_mailer.rb

class CampaignMailer < ActionMailer::Base
  
  def omg_notification(zomg)
    mail :to => # ...
  end

end

default_mailer.rb

class DefaultMailer < ActionMailer::Base
  include Sendgrid

  def notification(asd)
    sendgrid_mail :to => # ...
  end

end

sendgrid.rb

module Sendgrid
  def sendgrid_mail(*args)
    mail_message = mail(*args)
    mail_message.delivery_method.settings.merge!(APP_CONFIG[:sendgrid_config])
    mail_message
  end
end