SendGrid - Custom Event Notification App in header

I am trying to use SendGrid's Event Notification App (http://sendgrid.com/docum开发者_开发知识库entation/display/apps/EventNotification) for emails of certain categories. The Event Notification on my SendGrid account is empty. The header I'm putting on my email is:

X-SMTPAPI: {"category":"category","filters":{"eventnotification":{"settings":{"url":"theurl"}}}}

But I'm not getting a callback on my url. A simple curl post on this given url gives me the expected output, so I'm pretty sure that should be working with SendGrid too.

Do you have any ideas on what the header should look like? I couldn't find too much documentation on the website for this specific app...

Thanks!

Here is an example of the header that I am successfully sending with my Sendgrid emails:

headers("X-SMTPAPI" => "{\"unique_args\": {\"customer_id\":\"#{customer.id}\",\"email_batch_id\":\"#{batch_id}\"}, \"category\":\"monthly_statement\"}")

Make sure you have the correct events checked under the event notification settings in Sendgrid.