The two returning values in Action Mailer [bugfix]

Background

• Rails 7.1
• Ruby 3.x

Your production environment is sending emails using Action Mailer and smtp delivery method. You implemented a very common pattern using Action:

class NotificationMailer < ApplicationMailer
  def processed_success(user:)
    if user.pass_custom_condition?
      mail(to: user.email, subject: 'Record processed')
    else
      Rails.logger.info('Record processed but notification is not delivered')
    end
  end
end

Inside the Mailer, there is a condition that will return two possible values: a fully configured mail object and automatically is returned a nullable object ActionMailer::Base::MailNull.

The Iteration

An iteration that implements dynamic delivery settings.[1]

class NotificationMailer < ApplicationMailer
  include Customizable # concern to apply changes from db 

  def processed_success(user:)
    if user.pass_custom_condition?
      mail(to: user.email, subject: 'Record processed')
    else
      Rails.logger.info('Record processed but notification is not delivered')
    end
  end
end

Concern

module Customizable
  extend ActiveSupport::Concern

  included do # include callback into the mailers
    after_action :overrides_mail_settings, if: has_overrides_enabled?
  end

  def overrides_mail_settings
    mail.delivery_method.settings.merge!(@business.smtp_settings)
  end

  def has_overrides_enabled?
    @business.customize.present?
  end
end

The Bug

Following the suggested implementation in the rails documentation and the backward common pattern, you should receive this error when you try to deliver NotificationMailer#deliver_now

[ArgumentError] SMTP To address may not be blank: []

• Before the callback change, the mailing process finishes with a return value: Mail::Message or ActionMailer::Base::MailNull.
• After adding the callback, the mailer lifecycle expands with a message object copying default values into it.

Solution

Our goal is ensuring backward compatibility by returning: Mail::Message or ActionMailer::Base::MailNull.

Concern

module Customizable
  extend ActiveSupport::Concern

  included do # include callback into the mailers
    after_action :overrides_mail_settings, if: overrides_allowed?
  end

  def overrides_mail_settings
    mail.delivery_method.settings.merge!(@business.smtp_settings)
  end

  def overrides_allowed?
    @business.customize.present? && message.to.present?
  end
end

Final Words

Although this fixes our problem and ensures backward compatibility, I would rather move business validation out of the mailer. In my opinion, a mailer class is in the presentation layer responsible for configuring delivery details, customizing layouts and showing content. Any business rule should be in a different component like a service or job.

Bibliography

1. Action Mailer Basics — Ruby on Rails Guides. (2024). Ruby on Rails Guides. https://guides.rubyonrails.org/action_mailer_basics.html
2. Rails Api. (2025). rails/actionmailer/lib/action_mailer/base.rb at main · rails/rails. GitHub. https://github.com/rails/rails/blob/main/actionmailer/lib/action_mailer/base.rb

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