rescue_all_mailer_errors.rb

class UsersController ...
  def create
    @user = User.create(params[:user])
    Notifier.deliver_activation(@user)
    redirect_to :controller => :dashboard
  rescue Exception => e
    logger.error e
    ...
  end
end

rescue_quota_limit_exception.rb

class UsersController ...
  def create
    @user = User.create(params[:user])
    Notifier.deliver_activation(@user)
    flash[:notice] = "Check your email to active your account"
  rescue Heroku::Mailer::DailyQuotaReached
    logger.warn "Reached the daily limit of outgoing emails! Skipping activation"
    @user.activate
    flash[:notice] = 'Thank you, your account is ready :)'
  rescue Exception => e
    logger.error e
    ...
  end
end