hello_mailer.rb

class HelloMailer < ApplicationMailer

  def welcome(user)
    headers 'X-MSYS-API': { "options": { "open_tracking": true, "click_tracking": true } }.to_json

    mail(to: user.email, subject: 'Welcome!')
  end
end

welcome.html.erb

<h1>Hello There</h1>

<p>Check this out</p>
<!--this one will be converted to a tracking link -->
<a href="http://guides.rubyonrails.org/action_mailer_basics.html">Action Mailer Doc</a>


Another link:
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http://guides.rubyonrails.org/action_mailer_basics.html