This script should send emails by with categories and unique args.

rubyapi.rb

require '~/Dev/ruby/SmtpApiHeader.rb'
require 'mail'

Mail.defaults do
  delivery_method :smtp, { :address   => "smtp.sendgrid.net",
                           :port      => 587,
                           :domain    => "sendgrid.com",
                           :user_name => "sebfree", 
                           :password  => "*******",
                           :authentication => 'plain',
                           :enable_starttls_auto => true }
end
      
hdr = SmtpApiHeader.new

receiver = ['sebi.mihalache@gmail.com','sebi.mihalache@ymail.com']

hdr.addTo(receiver)
hdr.setUniqueArgs({'test' => 1, 'foo' =>2})
hdr.setCategory("testingapi")
a = hdr.as_string()
print a
print "\n"

mail = Mail.deliver do
  header['X-SMTPAPI'] = hdr.asJSON()
  to receiver
  from "seb@sendgrid.com"
  subject "Ruby API 2"
  text_part do
    body 'This tests the Ruby API'
  end
end