Sorry but I don't know a lot of the newsletter module to be more precise.
Please ask for more information.

The newsletter module doesn't send the mail to the eZ publish user because the
funtcion call $userSubscription = eZUserSubscriptionData::fetch( $subscription->attribute( 'email' ) ); in extension/ezxnewsletter/classes/ezsendnewsletteritem.php returns null because the data of a eZ publish user aren't in the ezxsubscriptionuserdata table even if the user has succesully completed the subscription to the newsletter list and is succesfully listed as subscriber.

This is becasue the function create in the eZSubscription class doesn't call eZUserSubscriptionData::create for a eZ publish user.