How to create custom email headers

I'm trying to create a custom email header to use the SendGrid api.

Here's what I'm doing - but its not working:

class Mailman < ActionMailer::Base
  default :from => "info@sample.com"

  def send_message(name, email, message)
    @name = name
    @email = email
    @message = message

    mail(:to => 'info@sample.com',
     :from => email,
     :subject => "Message from the site",
     :headers['X-SMTPAPI'] => "category: Drip Email"
    )
  end

end

Any help appreciated.

Thanks, Adam