I am not sure if I'm reading your email correctly. Are you trying to send the Registration Confirmation email following registration?
A couple of things you can check if you haven't already:
1. Does the email have advanced settings that might be preventing it from being mailed to that person such as the email is set to send to a specific reg type?
2. Here is a Cvent resource we have used in the past: Why was my contact opted-out? https://support.cvent.com/apex/CommunityArticle?id=000003176&Lang=en_US&searchTerm=opt%20outThis should help eliminate the reasons listed in the article and narrow down the reasons.
Hope this helps!