An example of sending custom headers with postmark-rails and ActionMailer

postmark_rails_headers.rb

class TestMailer < ActionMailer::Base

  def email
    headers['X-Tag-Via-Hash-Access'] = 'value'
    headers 'X-Tag-Via-Headers-Method' => 'value'

    mail(to: 'support@postmarkapp.com', from: 'tema@wildbit.com', subject: 'Headers test') do |format|
      format.text { render text: 'Headers test' }
    end

  end
end

msg = TestMailer.email
# => #<Mail::Message:2203334280, Multipart: false, Headers: <From: tema@wildbit.com>, <To: support@postmarkapp.com>, <Subject: Headers test>, <Mime-Version: 1.0>, <Content-Type: text/plain>, <X-Tag-Via-Hash-Access: value>, <X-Tag-Via-Headers-Method: value>> 

puts msg.to_postmark_hash['Headers'].inspect
# => [{"Name"=>"Mime-Version", "Value"=>"1.0"}, {"Name"=>"X-Tag-Via-Hash-Access", "Value"=>"value"}, {"Name"=>"X-Tag-Via-Headers-Method", "Value"=>"value"}]