Header handling doesn't match Mail gem's delivery methods

[cgriego]

When using any of the built-in Mail delivery methods, this works as expected to send a custom header.

class FooMailer < ApplicationMailer
  def foo_message
    headers['X-CUSTOM'] = "I am some header"
    mail(
      :from         => "example@example.com",
      :to           => "example@example.com",
      :subject      => "My Test message"
    ) do |format|
      format.text { "I am a message. Wheeeeeee" }
    end
  end
end

This also works.

class FooMailer < ApplicationMailer
  def foo_message
    mail(
      :from         => "example@example.com",
      :to           => "example@example.com",
      :subject      => "My Test message",
      "X-CUSTOM"    => "I am some header"
    ) do |format|
      format.text { "I am a message. Wheeeeeee" }
    end
  end
end

This gem is instead requiring this:

class FooMailer < ApplicationMailer
  def foo_message
    mail(
      :from         => "example@example.com",
      :to           => "example@example.com",
      :subject      => "My Test message",
      :headers      => {
        "X-CUSTOM"     => "I am some header"
      }
    ) do |format|
      format.text { "I am a message. Wheeeeeee" }
    end
  end
end

[tyrauber]

hi @cgriego,

The first method isn't an option because the gem builds upon the Mail instance and is unaware of any data not explicitly passed to it.

The second option is a possibility, but not a great one. The gem would need to be modified to assume any non-standard attribute applied to Mail is a Header. We'd need to explicitly state which attributes are not headers and filter them out. The branch feature/applied_headers shows how this would work. The risk would be accidentally exposing data to the headers that shouldn't be there.

Perhaps someone can come up with a cleaner, safer method?

[cgriego]

The two methods are really the same; ActionMailer essentially is just delegating headers to the Mail instance.

Things like 'To', 'From', 'Cc', 'Bcc', 'Reply-To', 'Subject', 'Content-Type' are all headers, but SendGrid's API treats them specially so handling them specially in the gem makes sense, like you have in the applied_headers branch. Then the other items are also SendGrid specific extensions like categories, personalizations, custom args, etc., so handling them this way also makes sense to me.

It's funny because I worry about accidentally exposing data the other way. In projects and/or environment not using the SendGrid delivery method, any data I put in headers or passed to mail is going to get sent in the email headers save for a few exceptions in the ActionMailer gem.

        assignable = headers.except(:parts_order, :content_type, :body, :template_name,
                                    :template_path, :delivery_method, :delivery_method_options)

If I were to use the exact same code I use with SendGrid with a different mailer (in say a dev, test, or qa environment), then my outgoing mail is going to expose stuff that sending with SendGrid would not. The mail sent with SendGrid will always expose less data than a built-in delivery method. Without patching ActionMailer or putting the SendGrid extensions into the delivery_method_options (which doesn't seem like a great fit), that will always be the case.