You could make a resolver function. In that function, you first query the user by the
event.context.auth to get the email address, then you get the Detail node for the Post from the PostId you pass in to the resolver. Compare email addresses, and if they match, return all fields from Post.
The only issue with this is that you cannot return nested Types, model Types, or relations from resolver functions yet.
The alternative would be to use an API Gateway in front of your GraphQL endpoint where you put that logic. Then you can return the actual Post node, so you can query all of its fields and relations from your client.